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M.O.225 ii (A.P. 897)
AIR
MINISTRY
METEOROLOGICAL OFFICE
THE
METEOROLOGICAL
GLOSSARY
THIRD
EDITION
In continuation of the Weather Map
Published by the Authority of the Meteorological Committee
LONDON: HIS MAJESTY'S STATIONERY OFFICE
1939
(Reprinted 1950 incorporating Amendment Lists Nos. 1-4)
Decimal Index
551-5 (03)
N. A.
METEOROLOGICAL GLOSSARY
CONTAINING INFORMATION IN EXPLANATION OF
TECHNICAL METEOROLOGICAL TERMS
The first edition of the Meteorological Glossary was issued in 1916, and
was reprinted with additions in 1917 and again in 1918. Since 1918 there
have been many advances in meteorology, for example in connexion with,
the study of weather maps, and the second edition issued in 1930 was almost
completely re-written, the task being apportioned among the Professional
Staff of the Office. In this second edition the opportunity was taken, by
increasing the size of the page, to bring the Glossary into conformity with
the other handbooks published by the Meteorological Office.
In accordance with the practice of the Oxford Dictionary the initial word
of each article is in black type. Words in small capitals in the body of the
text are the subjects of articles in another part of the Glossary. International
definitions of cloud forms are distinguished by the use of italic type.
The International Meteorological Committee at the meeting in London
in 1921 passed a resolution requesting the inclusion, in a future edition of the
Meteorological Glossary, of the equivalents in various languages of the words
defined. In the second edition effect was given to this resolution by the
inclusion at the end of the work of equivalents in Danish, Dutch, French,
German, Italian, Norwegian, Portuguese, Spanish and Swedish.
The progress of meteorological knowledge since 1930 has again made a
number of revisions necessary, and the publication of a third edition has
given an opportunity of incorporating these. Thanks are due to all those,
both the staff of the Meteorological Office and others, who have given help
in the revision.
Meteorological Office, Air Ministry.
November, 1938.
METEOROLOGICAL GLOSSARY
Absolute Extremes.—See EXTREMES.
Absolute Humidity, also called vapour density, is the mass of aqueous
vapour per unit volume of air, and is usually expressed in grammes per cubic
metre of air. The term is sometimes incorrectly applied to the pressure of
the aqueous vapour in the air. In accordance with Dalton's Law the water
vapour in the atmosphere exerts the same pressure as it would if the air
were not present. Knowing the vapour pressure e, we therefore obtain d,
the mass of vapour in a cubic metre of moist air, from the formula
where <50 is the density of aqueous vapour under standard pressure and
temperature P0 and T0
grams per cubic metre.
whence d = 216-7
e being measured in millibars and T in absolute degrees on the tercentesimal
scale. See AQUEOUS VAPOUR.
Absolute Temperature. The absolute scale of temperature is formulated
by reasoning about the production of mechanical work at the expense of heat
(which is the special province of thermodynamics, see ENTROPY). The
absolute temperatures corresponding with 0° C. and 100° C. are 273-16 and
373 16 respectively. For practical purposes the scale may be taken as
identical with that based on the change of volume and pressure of one of the
permanent gases with heat. It is moreover sufficiently accurate to replace it
by a scale (to which the name TERCENTESIMAL has been given) obtained by
adding 273 to the Centigrade temperature. It is usual in meteorology to
use temperatures on this scale in formulae which are strictly true only for
absolute temperatures, such temperatures being frequently indicated by
°A. (thus 10° C. = 283° A.), a convention that has been adopted in the present
edition of the Glossary. For thermometric purposes aiming at the highest
degree of accuracy the hydrogen scale is used, but for the purposes of
meteorological reckoning the differences of behaviour of the permanent gases,
hydrogen, oxygen, nitrogen are unimportant. In physical calculations for
meteorological purposes the absolute is the natural scale ; the densities of air
at any two temperatures on the absolute scale are inversely proportional to
the temperatures. Thus the common formula for a gas,
Po
P
po (273 + /J
p (273 + /)
where p is the pressure, p the density and t the temperature Centigrade of
the gas at one time, p0, p0, tQ the corresponding values at another, becomes
.
?T
Po T0'
where T and TQ are the temperatures on the absolute Centigrade scale. Its
most important feature for practical meteorology is that from its definition
there can be no negative temperatures. The zero of the absolute scale is the
temperature at which all that we call heat would have been spent. In the
Centigrade scale all temperatures below the freezing point of water have to
. This is very inconvenient, especially
be prefixed by the negative sign
for recording observations in the upper air, which never gives temperatures
above the freezing point in our latitudes at much above 4 kilometres (13,000 ft.),
and often gives temperatures below the freezing point nearer the surface.
The absolute temperature comes into meteorology in other ways ; for
example, the rate at which heat goes out into space from the earth depends,
according to Stefan's Law, upon the fourth power of the absolute temperature
of the radiant substance. See RADIATION.
(90565)
A2
ACCELERATION
4
Absolute temperature on the Fahrenheit scale is degrees Fahrenheit plus
459-4. The Absolute Fahrenheit scale is sometimes referred to as the
Rankin Scale.
Acceleration the rate at which velocity changes with time. Any
convenient unit such as the mile per hour per hour or the centimetre per
second per second may be used, its dimensions being length/square of time.
Acceleration, like velocity, is a vector having both magnitude and direction.
Thus in uniform circular motion, although the speed does not change the
direction does, and the motion has an acceleration directed towards the centre
of rotation the so-called centripetal acceleration.
In meteorology we are mainly concerned with the acceleration of the moving
atmosphere and, in accordance with the fundamental principle of dynamics,
we may write down an expression for the acceleration by equating it to the
vector sum of all the forces acting upon unit mass of air. The only real forces
concerned are the force of gravity and the gradient of pressure both of which
are theoretically determinable by observation ; but, in small scale motions,
such as convection currents and squalls, where vertical accelerations are
significant, the difficulties are such that no attempt has yet been made to
solve the equations of motion even approximately. Large scale motions
are more amenable to mathematical analysis because then the vertical
acceleration is entirely negligible compared with the acceleration of gravity ;
the force of gravity may be cancelled against the vertical gradient of pressure.
The only acceleration of significance is in that case the horizontal component
and the only remaining force is the horizontal gradient of pressure. There
are however two decisive reasons why we cannot determine the acceleration
from the force in the atmospheric problem. First there is the effect of
turbulence. The motion at any level is affected, through the process of mixing,
by the motion both above and below. The result is in a sense analogous to
that of viscosity in a non-turbulent fluid and may be allowed for by introducing
a " frictional term " into the equation of motion. Near the earth's surface
this term is important and gives a large contribution to the acceleration,
but within the free atmosphere it may justifiably be ignored except perhaps
in regions of rapid sheer. The second factor is the effect of the rotation of the
earth. The observed acceleration is referred to axes rotating, in space, and
this must be allowed for by including the " geostrophic acceleration " which is
proportional to the velocity and acts at right angles to the direction of motion.
Ignoring turbulence the apparent acceleration of the air may be equated
to the acceleration due to the pressure gradient plus that due to the earth's
rotation. We may write
dt
p
where V is the vector wind, G is the horizontal gradient of pressure taken
positive towards the low pressure, p is air density, I the geostrophic factor
2o> sin (j> (see GRADIENT WIND) and i signifies rotation of the vector IV through
one right angle in the cyclonic direction. The vector equation may be
expressed by the triangle of accelerations shown in Fig. 1 where V is the,
velocity, IV is at right angles to V, G/p is directed towards the low pressure
and
dt
is the acceleration.
This equation, it should be noted, does not provide a method of determining
acceleration from the pressure gradient, but merely gives a relationship between
acceleration and velocity ; theoretically, for a given gradient, the acceleration
may have any arbitrary value if the velocity is suitably adjusted. To make
any progress it is therefore necessary first to make -some assumption as to
the nature of the motion. For example we might assume zero acceleration
when,
UV = G/p
............ (2)
5
ACCUMULATED TEMPERATURE
which is the familiar geostrophic wind equation. This gives a useful first
approximation to the true wind, but of course gives no clue to the general
problem since the essential factor, acceleration, has been ignored.
Alternatively we might assume uniform circular motion, when equation (1)
immediately reduces to the so-called gradient wind equation. This also is
of some value but as it implies that only the direction and not the speed of
the wind is changing, or in other words the kinetic energy is constant, it has
no application to the fundamental problems. The most fruitful results have
been obtained by developing equation (1) and ignoring certain derivatives of
higher order. Brunt and Douglas* in this way arrived at what is known as
the isallobaric acceleration, proportional to the isallobaric gradient and
directed along the isallobars. More recently Sutcliffef has re-examined the
problem and, removing certain restrictions made by Brunt and Douglas,
has shown that the isallobaric acceleration is only one of four terms. Further
work is however required to decide finally which of these is dynamically
significant.
G
P
The determination of the field of acceleration is important not only directly
but because it has certain interesting properties. Equation (1) might be
written, by using equation (2),
dt
where V is the departure of the wind from the geostrophic value. Now
geostrophic winds are rigorously non-divergent so that the field of convergence
and divergence is given by the field of V, that is of acceleration. Since
convergence and divergence alone give rise to vertical currents with the
accompanying cloud and rain and also alone are the cause of the development
of cyclonic or anticyclonic circulation, a study of the field of acceleration
should go a long way towards a final solution of the general problem of
depressions and anticyclones.
Absorption (Atmospheric).
See RADIATION (Radiation and the atmosphere) .
Accumulated Temperature. The integrated excess or deficiency of temperature measured with reference to a fixed datum over an extended period of time.
If on a given day the temperature is above the datum value for n hours and the
mean temperature during that period exceeds the datum value by m degrees the
accumulated temperature for the day above the datum is nm hour-degrees
or nm/24 day-degrees. By summing the daily entries arrived at in this way,
the accumulated temperature, above or below the datum value, may be
evaluated for periods such as a week, a month, a season or a year. In practice,
* London, Mem. R. met. Soc., 3, 1929, No. 22.
t London, Quart. J. R. met. Soc., 64, 1938, p. 495-504.
ACCURACY
6
hourly values are not employed, and the daily entries are deduced from formulae
based on readings of the maximum temperature, X, and the minimum
temperature, N. It is sufficiently accurate to assume that the mean temperature for the day is equal to J (X -f- N). If during a whole day the temperature
remains continuously above the datum temperature D (i.e. if both X and N
are greater than D) then clearly the accumulated temperature above D is
equal to £ (X + N)
D, and the accumulated temperature below D is zero.
Similarly if both X and N are less than D, the accumulated temperature
above D is zero and the accumulated temperature below D is equal to
D
\ (X + N). When D is intermediate between X and N empirical
formulae of greater complexity must be used. For the purposes of the Weekly
Weather Report the value of D is taken to be 42° F. and the following
formulae are used :
Accumulated temperature for the day expressed
in day-degrees (Fahrenheit)
Cases
Above 42° F.
(X - 42) > (42 - N)
(X - 42) < (42 - N)
(X - 42) - i (42 - N)
i (X - 42)
Below 42° F.
i (42 - N)
(42 - N) - J (X - 42)
The practice of arriving at weekly accumulations by summing daily values
obtained in this manner dates from the year 1928. Prior to that time weekly
values were deduced directly from weekly means of daily maximum and
minimum temperatures by the use of formulae due to Lt.-Gen. Sir Richard
Strachey. For further information reference may be made to the Introduction
to the Weekly Weather Report, Vol. XLV (New Series), 1928-9 and to
Form 3300 which contains tables, based on the above formulae, to eliminate
computation.
The adoption of 42° F. (6° C.) as the basic temperature follows the practice
of Angot and others who have considered that value to be the critical
temperature above which the growth of vegetation in a European climate
is initiated and maintained. For the study of heating problems a basic
temperature of 60° F. has been adopted by American and British engineers.
Accuracy. Strictly defined, the term accuracy signifies exactitude. No
physical quantity, however, can be measured with exact precision. If a
number of measurements are made of the quantity, every care being taken to
eliminate all known sources of error, the final results will still differ amongst
themselves. The deviation of these values from the true value is due to the
accumulated sum of numerous small errors of unknown origin which cannot
be allowed for. Hence in physical work the word accuracy is used in a
comparative rather than in an absolute sense. It indicates the closeness with
which an observation of a quantity is considered to approach the unknown
true value of that quantity. A considerable body of mathematical doctrine
has been built up dealing with this subject: it is usually called the Theory
of Errors.
See also ERROR.
Actinic Rays. Radiation which is effective in bringing about chemical
changes, as in photography.
Actinometer. An early name for an instrument measuring the rate at
which radiation is received from the sun (see PYRHELIOMETER). More
recently the same name has been given to instruments for measuring the
intensity of actinic rays.
Action Centre. See CENTRE OF ACTION.
ADIABATIC
7
Adiabatic. The word which is applied in the science of thermodynamics
to the corresponding changes which may take place in the pressure and
density of a substance when no heat can be communicated to it or withdrawn
from it.
In ordinary life we are accustomed to consider that when the temperature
of a body rises it is because it takes in heat from a fire, from the sun or from
some other source, but in the science of thermodynamics it is found to be
best to consider the changes which occur when a substance is compressed
or expanded without any possibility of heat getting to it or away from it.
In the atmosphere such a state of things is practically realised in the interior
of a mass of air which is rising to a position of lower pressure, or sinking to one
of higher pressure. There is, in consequence, a change of temperature which
is called mechanical or dynamical, and which must be regarded as one of the
most vital of meteorological phenomena because it accounts largely for the
formation and disappearance of cloud, and probably for the whole of our
rainfall.
Tyndall illustrated the change of temperature due to sudden compression
by pushing in the piston of a closed glass syringe and thus igniting a piece
of tinder in the syringe. The heating of a bicycle pump is a common
experience due to the same cause.* On the other hand the refrigeration of
air is often obtained simply by expansion, particularly in the free atmosphere.
To plan out the changes of temperature of a substance under compression
and rarefaction alone, we have to suppose the substance enclosed in a case
impermeable to heat the word adiabatic has been coined to denote
impermeable to heat in that sense. The changes of temperature thereby
produced are very great, for example :
For adiabatic change of pressure
decreasing from 1,000 mb. by
The fall of temperature from
290° A., 62-6° F., is
in.
mb.
10 or 0-30
2-95
100
5-91
200
8-86
300
11-81
400
500
14-77
17-72
600
20-67
700
23-62
800
26-58
900
° F.
0 C.
1-6
0-9 or
15-7
8-7
32-8
18-2
51-1
28-4
71-8
39-9
95-0
52-8
67-6
121-7
85-5
153-9
194-6
108-1
254-3
141-3
The adiabatic lapse rate of dry air is given by the exp rcooioii v — i g__
R
y
where R = gas constant = 2-8703 x 106
g = acceleration of gravity = 980-617 cm./sec. 2 in latitude 45° at
mean sea level
y = the ratio of the specific heats of dry air at constant pressure
and constant volume respectively = 1 402
from which the lapse rate in latitude 45° at M.S.L. is 0-98° C. per hundred
metres.
In the case of moist air matters are greatly complicated by the fact that
when rising air cools to such an extent that condensation of water vapour
takes place, the fall of temperature is checked by the latent heat liberated.
In saturated ascending air there is in consequence a lower lapse rate, and one
which varies greatly with the temperature. Two kinds of moist adiabatics
are distinguished, one (reversible) in which the condensed water, whether in
* Dangerous heating may result on firing a gun from sudden compression of gas within
the bursting charge of the shell if there are cavities in the explosive therein.
ADVECTION
8
the form of rain, hail or snow, is retained, and the other (irreversible) in which
it falls out of the air. In the latter case the air on descending would follow
the dry adiabatic relationship. In the diagram below the dotted lines are
reversible adiabatics*.
500 Absolu fe
590
2SO
270
2tO
I ni i) n M I i i i i I i i i i I i t n I I i I i I i i I I I i i ii I i I I 11 MOIU '
.000
|TT, I 1 i i i 11 I I i i p i 11 1 i ' i ' | ' ' ' ' | I i I
i ' ' ' I I I ' ' I ] ' ' I
FIG. 2. Diagram showing the pressure in the upper air corresponding
with the standard pressure (1013-2 mb.) at the surface and adiabatic lines
for saturated air referred to height and temperature. (From Neuhoff,
Smithsonian Miscellaneous Collections, Vol. 51, No. 4, 1910.)
The pressure is shown by full lines crossing the diagram, and the
adiabatic lines for saturated air by dotted lines. Temperatures are given
in the absolute scale.
The short full lines between the ground and the level of 1,000 metres
show the direction of the adiabatic lines for dry air.
Advection. The process of transfer by horizontal motion. The term
is more particularly applied to the transfer of heat by horizontal motion of
the air. The transfer of heat from low to high latitudes is the most obvious
example of advection.
Aerodynamics. The study of the forces and reactions arising from the
motion of bodies, more particularly the parts of an aeroplane, through the
air. It is from such forces or reactions that the upward pressure beneath
the wings of an aeroplane, which provides the lifting power, is obtained.
Aerology. A word denoting the study of the atmosphere, and including
the upper air as well as the more general studies understood by the word
METEOROLOGY. It is frequently used as limiting the study to the upper air.
After Glow. See ALPINE GLOW.
* A table giving the saturated adiabatic lapse rate for different layers of the atmosphere
is on p. 224.
9
AIR MASS
Air. The mixture of gases which form the atmosphere. Air from which
the dust and water vapour have been removed shows no measurable changes
in composition from place to place, within the limits of height accessible to
direct observation. The dust is regarded as an impurity and the water,
whether in the form of vapour or cloud, as an addition to the air. For
meteorological purposes we may therefore treat dry air as a uniform mixture.
Its composition is shown in the following table the figures in the penultimate
column being those given by Paneth* :
Proportional composition
Specific gravity
(0 = 16)
By volume
Dry air
Nitrogen
Oxygen
Argon . .
Carbon dioxide
Neon . .
Helium
Krypton
Hydrogen
Xenon . .
Ozone . .
14-48
14-01
16-00
19-94
22-15
10-1
1-99
41-5
1-008
65
24
o/
/o
100-0
78-09
20-95
0-93
0-03
0-0018
0-0005
0-0001
0-00005
8 x 10-«
1 x 10-«
By weight
<V
/o
100-0
75-54
23-14
1-27
0-05
0-0012
0-00007
0-0003
0-000004
3-6 x 10-«
1-7 x 10-«
According to Paneth these percentages are practically the same throughout
the troposphere and even for some distance into the stratosphere except in
the case of ozone, the amount of which is variable and normally increases
with height, while another gas of variable amount radon the percentage
by volume of which is only about 6 X 10~ 18 decreases with height.
Dry atmospheric air at 0° C. and 1,000 mb. has a density of 1 -27617 X 10-3
grams per cubic centimetre. The ratio of the density of water vapour to
that of dry air at the same pressure and temperature is 0-6221, or very
nearly f. If v is the volume in cubic centimetres of one gram of air, p the
pressure in millibars, and T the temperature on the absolute scale, these
quantities are related by the equation p v =RT, where R = 2 • 8703 X 10".
The number of molecules in one cubic centimetre of dry air at 0° C. and 760 mm.,
or 1013 2 mb. is 2 75 X 10 19. The specific heat of dry air at constant volume
(cv) is 0-1715, and the specific heat at constant pressure (cv) is 0-2417. The
ratio of the specific heats is 1 -402. The thermal conductivity of air is very
low, being 5 6 X 10~5 in C.G.S. units. See ADIABATIC.
Air is slightly viscous, the coefficient of viscosity, v, being 1 7 X 10~4 at
0° C. and 2-2 x 10-* at 100° C.
Air Mass. Broadly this term is used in synoptic meteorology to denote
the mass of air which is bounded by frontal surfaces (often smoothed out
into transitional zones), and often extending in well marked examples from the
ground to the stratosphere. The horizontal dimensions are normally hundreds
or even thousands of miles. The term may, however, legitimately be used in
describing phenomena on a much smaller scale. The conception of large scale
air masses was introduced in connexion with TRAJECTORIES and has been
greatly developed in connexion with FRONTS and related questions. Its
practical utility depends on two observed characteristics of the atmosphere.
In the first place, although the temperature and humidity of the ak in contact
with a land or sea surface are much influenced by that surface, the upper air
characteristics are relatively conservative throughout a motion of many
hundreds of miles. When there is vertical motion the air is affected by
ADIABATIC processes, but if the air is followed in three dimensions the
* London, Quart. J. R. met. Soc., 63, 1937, p. 436.
(90565)
A*
AIR MASS
10
POTENTIAL TEMPERATURE and the water content per unit mass are independent
of adiabatic processes in the absence of condensation or evaporation, and the
WET-BULB POTENTIAL TEMPERATURE remains constant even when these are
included. This function is therefore especially useful in identifying air masses.
The non-adiabatic processes, turbulence and radiation, are slow in their
operation in the upper air.
The other important feature is that the air is often nearly homogeneous
over a large area, the horizontal differences being concentrated in the frontal
zones. It is well known that a steep horizontal gradient of temperature involves
a large change of wind with height. Such a sheering action distorts an air
mass out of recognition, and readily leads to mixing between air masses
originally adjacent to each other. When one is dealing with air masses
hundreds of miles in linear dimensions, horizontal diffusivity is not very
effective, but distortion due to marked wind variation with height brings
in the more effective process of diffusion in the vertical, due to the
TURBULENCE which wind sheering tends to produce. Though these processes
often limit the application of the idea of air mass, there are fortunately many
cases of a nearly homogeneous mass, by which is implied a weak horizontal
temperature gradient. This feature depends on various factors, involving the
history of the air over periods extending to weeks. Regions where homogeneous
air masses tend to be produced are known as " source regions." Uniformity
of surface conditions is a favourable feature, since the surface influence is
diffused slowly upwards. A quiet state of the atmosphere is another favourable
factor. In the area of the sub-tropical " highs " both conditions are satisfied,
and those regions are the source of TROPICAL AIR, which is the most generally
homogeneous type of air (except quite near the fronts). In the arctic regions
there are often rapid fluctuations of pressure and it is unlikely that the upper
air is really homogeneous, but all air of arctic origin, until it has subsided,
is cold aloft and is often found to be homogeneous over a reasonably large
area. The term POLAR AIR is a wide one, implying a source region which
includes areas of cold water in addition to ice-covered areas, but in winter
most examples of polar air can be traced back to ice or snow-covered surfaces.
Polar air in its various subdivisions is the most frequent type of air in our
area, at least in the lowest 15,000 to 20,000 ft., partly because warm air is
often displaced upwards, and also (in winter) because of circulation round the
" Iceland low." Its characteristics are far more variable than those of
tropical air.
Homogeneity in the air mass is also affected by vertical motion outside
the " source region." In particular cold air tends to subside and be
dynamically warmed, simultaneously with the formation of an anticyclone,
and within an air mass of polar origin there is often a considerable gradient
of temperature between an anticyclone or wedge and the rear of a depression.
In the life history of a normal depression there are originally two very different
air masses, but in the final stage a high degree of homogeneity is often
attained. Both persistent (warm) anticyclones and old (cold) depressions are
sources of homogeneous air.
In considering the classification of air masses it is important to note that
there are no clearly defined categories which cover the infinite variations of
nature, but that a classification is imposed for practical convenience. The
particular system of classification must be based on the needs of each area,
but normally the emphasis is on the relatively distant " source regions," which
constitute the most important single factor, in spite of the other influences
mentioned above, and also the complication of local surface conditions. In
the British Isles the main categories are tropical and polar air ; ARCTIC AIR
is really a special type of polar air. Further details may be found under these
headings. The two main categories are sub-divided into Maritime and
Continental. In the British Isles air of a really continental source is rare.
Air coming back from Europe is usually maritime in origin, and its varying
length of continental life history is merely a complicating factor. The
11
ALBEDO
expression " continental air " (without adding polar or tropical) is occasionally
used, and there are also occasions, especially in summer, when an unqualified
" maritime air " might be the best description.
Generally speaking, air-mass analysis is simplest in winter, when the
exchange of air between high and low latitudes is most marked. In addition
the continents are cold in winter in temperate latitudes, so that if air moving
from the continents over the oceans is included in polar air, one has only
the two main categories applicable to the Atlantic and to most conditions in
the British Isles. The lapse rate of temperature in the lowest few thousand
feet is the main distinguishing feature, and this is important in relation to
cloud height and also to the height at which ice can form on aeroplanes. The
difference between sea and air temperatures observed from ships is a useful
practical criterion, but it should not be used too rigidly. In summer, when the
continents are warm to about latitude 65° N, air moving from the continents
to the oceans is cooled at the surface and becomes stable and damp in the lower
layers. Such cases, which are by no means rare, are not adequately covered
by any classification in general use.
Air-Mass Climatology. With the passage of depressions and anticyclones
any place in the temperate storm belt is successively covered by air masses of
different origins. Five main types of air mass are recognised, Polar Maritime
(PM), Polar Continental (PC), Tropical (or Equatorial) Maritime (TM),
Tropical Continental (TC) and Indifferent or Mixed (X). In any locality and
season each of these types has its own special characteristics, in England in
spring for example Polar Maritime air is cool and unstable, giving moderate
precipitation in the form of rain or hail showers. Polar Continental air is
cold and dry, Tropical Maritime air is warm and moist. This type of climate
is termed " collective " ; the normal values of temperature, humidity, etc.,
give the result of averaging out the characteristics of the different ah- masses.
A different and in some ways a more illuminating picture of the climate is
obtained by setting out the frequencies of the different types of air mass and
the characteristics of each type. This is " air-mass climatology." The
subject has been extensively studied in Germany, see for example Dinies, E.
" Luftkorper-Klimatologie," Hamburg, Arch. Seewarte, 50, No. 6, 1932.
Air-meter. An instrument for measuring the flow of air. It consists
of a light " windmill " in which inclined vanes are carried on the spokes
of a wheel arranged to rotate about a horizontal axis. A system of counters
is provided to show the number of rotations of the wheel. It forms a
convenient portable ANEMOMETER for use in winds of any strength, but a
meter designed for use in light winds would be damaged if exposed to strong
winds.
Air Pockets. Regions of relatively descending air, upon entering which
an aircraft experiences a proportionate decrease of lift. Since air streams
tend to follow the normal contour of the earth's surface, descending currents
will naturally exist on the leeward side of hills, buildings and other obstructions.
These descending currents will vary with the strength and character of the
wind, and with strong and squally winds they are frequently considerable.
The turbulence produced by an obstacle to the flow of the wind extends to a
height depending on the lapse rate, but this height is seldom more than
three or four times that of the obstacle. Well defined regions of descending
air will be experienced in the turbulent conditions associated with thunderstorms, squalls and fronts of cold front type. See BUMPINESS.
Air Trajectory. See TRAJECTORY.
Albedo. The proportion of the radiation falling upon a non-luminous
body which it diffusely reflects. The albedo of the earth is about 0-4, which
means that four-tenths of the sun's radiation is reflected to space. The
(90565)
A
2
ALPINE GLOW
12
greater part of this reflection is due to the clouds, which, according to
Aldrich,* have an albedo of about 0-78, but varying with the type and
thickness of the cloud.
Alpine Glow. A series of phenomena seen in mountainous regions about
sunrise and sunset.
Two principal phases are generally recognised
(a) The true Alpine glow.—At sunset this phase begins when the sun is
about 2° above the horizon ; snow-covered mountains in the east are seen to
assume a series of tints from yellow to pink, and finally purple. As this
phase is due mostly to direct illumination by the sun it terminates when the
mountain tops pass into the earth's shadow. The Alpine glow is most striking
when there are clouds in the western sky and the illumination of the mountains
is intermittent.
(b) The after glow begins when the sun is well below the horizon, 3° or 4°.
The lighting is faint and diffuse with no sharp boundary. It is said to occur
only when the PURPLE LIGHT is manifest in the opposite sky.
Altimeter. An aneroid barometer graduated to show approximate height
instead of pressure. If p0 is the pressure on the ground and p the pressure
at height h, then h can be calculated if the temperature of the air at every
point between p0 and p is known. Altimeters are generally designed so that
for a certain distribution of temperature with height the movement of the
pointer is proportional to the change of height, and the dial is made adjustable
so that the pointer can be made to show 0 when the instrument is at ground
level. One of two arbitrary assumptions about the temperature of the air
is adopted in this country.
(1) That temperature is 10°C. (50°F.) at aU heights, or
(2) That the distribution of temperature with height is that of the 1 .C.A.N.
standard atmosphere (see STANDARD ATMOSPHERE).
Altitude. The angular distance of an object from the horizon, measured
in a vertical plane. In meteorology, altitude generally means height above
mean sea level. It is usually expressed in metres or feet, but for calculations
involving pressures or other forces which depend on the value of GRAVITY it is
often more convenient to adopt as a unit the " dynamic metre," equivalent
to the standard metre multiplied by 1000/g. Surfaces of equal altitude in
dynamic metres are surfaces of equal geopotential.
Altocumulus (Ac.). -A layer, or patches composed of laminae or rather
flattened globular masses, the smallest elements of the regularly arranged layer
being fairly small and thin, with or without shading.
These elements are
arranged in groups, in lines or waves, following one or two directions and are
sometimes so close together that their edges join.
The thin and translucent edges of the elements often show irisations
which are rather characteristic of this class of cloud.
An important variety is known as altocumulus castellatus. The grouping
of the cloudlets is similar to that of ordinary altocumulus, but many of them
develop turreted tops like miniature cumulus. When seen moving from a
southerly point during fine weather these clouds indicate an early change to
thundery conditions. See CLOUDS.
Altostratus (As.). Striated or fibrous veil, more or less grey or bluish in
colour. This cloud is like thick cirrostratus but without halo phenomena ; the
sun or moon shows vaguely, with a gleam, as though through ground glass.
Sometimes the sheet is thin with forms intermediate with cirrostratus (altostratus
translucidus}. Sometimes it is very thick and dark (altostratus opacus), sometimes
even completely hiding the sun or moon. In this case differences of thickness
* Washington D.C., Smithson- misc. Coll., 69, No. 10, 1919.
13
ANEROID BAROMETER
may cause relatively light patches between very dark parts ; but the surface never
shows real relief, and the striated or fibrous structure is always seen in places
in the body of the cloud.
Amplitude. The amplitude of a harmonic motion is the maximum swing
to either side of the mean position. A harmonic motion can be represented
by a sine curve Rsinpx, where x is the independent variable and R is the
amplitude of the function, or of the motion which it represents. It should
be noted that the total range of motion is twice the amplitude. See
HARMONIC ANALYSIS.
Anabatic. Referring to the upward motion of air due to CONVECTION.
A local wind is called anabatic if it is caused by the convection of heated
air : as, for example, the breeze that is supposed to blow up valleys when the
sun warms the ground. See VALLEY BREEZE.
Anemogram. The record of an ANEMOGRAPH.
Anemograph. An instrument for recording the velocity or force, and
sometimes also the direction, of the wind. The best known forms are the
Robinson cup anemograph, the Dines tube anemograph, the " anemobiagraph "
designed by Halliwell for Negretti and Zambra, the Osier pressure-plate
anemograph and the Richard " anemo-cinemograph."
The Robinson cup anemograph depends for its action on the rotation,
under the action of the wind, of a group of hemispherical cups carried on
arms attached to a vertical spindle. There is a nearly constant ratio (known
as the " factor ") between the travel of the wind and the travel of the cups.
This factor depends upon the dimensions of the instrument and must be
determined by test in a wind tunnel; it is more nearly a true constant for
the 3-cup than for the 4-cup anemometer.
The Dines anemograph makes use of the difference of pressure set up
between two pipes, one of which is kept facing the wind, while the other is
connected to a system of " suction holes " on a vertical tube. The difference
of pressure so produced is arranged to raise a float carrying a pen, the height
of which above the zero position is made proportional to the wind speed.
By this method every gust and lull may be shown on the record. The
anemobiagraph works on similar principles.
The pressure-plate anemograph records the pressure of the wind on a
vertical plate facing the wind, while the anemo-cinemograph records, by
means of an ingenious system of electrical transmission, the rate of rotation
of a cup system.
Anemometer. An instrument for determining the velocity or force of the
wind. As in the case of ANEMOGRAPHS various types are in use, the most
important being those in which a system of cups is employed. In the
Meteorological Office pattern " cup indicating " anemometer, the rotation
of the cups operates a train of wheels designed to indicate the total flow of
air in miles. In the " cup electric " anemometer an electrical circuit containing
a buzzer is arranged to give an audible signal for a fixed flow of air. Similar
anemometers have been designed to indicate the rate of rotation of the cups,
and, therefore, the air velocity by means of a speed indicator.
Anemoscope. An instrument for indicating the existence of wind and
showing its direction.
Aneroid Barometer. The aneroid barometer was invented by Lucien
Vidie in about 1843. In its simplest form it consists of a shallow capsule of
thin corrugated metal which is very nearly exhausted of air. The faces are
kept apart by a spring. The residual air provides an automatic correction
for temperatures. In some instruments a number of capsules are employed.
The relative movements of the faces due to changes of atmospheric pressure
are conveyed through a train of levers to a chain which actuates a pointer
on a dial. An aneroid is light, portable and convenient, but should be
compared occasionally with a mercury barometer, as an appreciable change
ANEROIDOGRAPH
14
of zero sometimes occurs. Owing to errors introduced by the imperfect
elasticity of the metal, etc., the use of aneroid barometers in meteorology is
confined chiefly to cases where mercurial instruments are impracticable,
e.g. on aircraft or in the construction of self-recording instruments where the
highest order of accuracy is not expected. Owing to improved methods of
manufacture, considerable progress has been made in recent years towards the
production of aneroids comparable in accuracy with mercurial barometers.
Aneroidograph. A self-recording aneroid. An aneroid barometer provided
with mechanism for recording the variations of pressure of the atmosphere.
See BAROGRAPH.
Angular Velocity. The angular velocity of a moving line is the rate of
change of the angle between the line and a fixed line in the same plane.
A suitable convention is adopted as to which direction of rotation shall be
considered positive. The angular velocity of a moving point about a fixed
point is the angular velocity of the line joining the two points. The angular
velocity of a moving point about a fixed axis, is the rate of change of the
angle between a plane drawn through the fixed axis and the moving point,
and a fixed plane passing through the axis ; and the angular velocity of a
solid body about an axis is the angular velocity of any point of-the solid body
about that axis. Angular velocity may be measured in revolutions per minute
or per second ; for many purposes it is more convenient to measure it in radians
per second or per hour, the symbol for it being at. Since there are 2 n radians
in a complete revolution, the connexion between <o and the revolutions per
minute, N, is CD = nN/3Q radians per second. The linear velocity of a cloud
is determined as a product of the angular velocity of the cloud about the point
of the earth's surface vertically beneath it and the height of the cloud above
that point. If the height of the cloud is given in miles and the angular velocity
in radians per hour the linear velocity will be in miles per hour.
Anomalous Radio Propagation. This term refers to the refraction by the
atmosphere of radio waves of wave-lengths of a few metres or less transmitted
in beams (e.g. radar) at a very small angle to the horizontal. " Superrefraction " is also used. Normally the waves are only refracted to a small
extent and the transmission is practically the same as for light waves.
When, however, there is an inversion of temperature or a rapid decrease of
water vapour content with height there is considerable downward refraction
which may be sufficient to make the waves follow the curvature of the earth
and give transmission to far greater ranges than is normally possible. The
most favourable conditions for long ranges occur in air moving out from a
warm dry land over a cool sea. Thus in the NE. monsoon season of India
very great radar ranges are obtained.
Anomaly. The departure of a meteorological element from its normal
value. In meteorology the word is used chiefly in connexion with temperature,
to indicate the departure from the mean value for the latitude. Places that
are relatively warm for their latitude, such as the Norwegian coast in winter,
have a positive temperature anomaly; places that are relatively cold, such
as Siberia in winter, have a negative temperature anomaly.
Anthelion. A colourless mock sun appearing at the point of the sky
opposite to and at the same altitude as the sun. The phenomenon is rare.
Rather more frequently oblique arcs crossing at that point are reported.
The phenomena are no doubt due to the reflection of light from ice crystals,
but the exact explanation is in doubt.
Anticyclone. An anticyclone is a region in which the barometric pressure
is high relative to its surroundings (Plate I). In temperate latitudes anticyclones are usually isolated and are generally distinguished by a series of
closed isobars of roughly circular or oval form, the region of highest pressure
being the central region of the anticyclone. The term " anticyclone " was
first introduced by Sir Francis Galton in 1861 with the intention not only
Plate!.
to fa.ce Page 14
.Q
48
,c/pr
1004
°JP Low
1024
Low
1020
JULY 17, 1928, 7h.
ANTICYCLONE
3254-3034- MI3I5- 1875-8/50 (Ty.P)
15
ANTICYCLONE
of conveying the idea that it was an area of high pressure instead of low
pressure as in a depression or cyclone, but also that the general weather
in the two systems, that of high and low pressure, was entirely different in
character. In the depression the wind circulation is, in the northern
hemisphere, counter-clockwise, often approaching gale force, and the weather
is usually of an unsettled type while in an anticyclone the wind circulation
is clockwise and the weather of a quiet and settled type.
There are two extensive belts of anticyclones, approximately in lat.
30° N. and 30° S. ; these almost completely encircle the earth and are more
or less permanent; they usually drift a little northwards and a little southwards
as the sun moves northwards and southwards in its path across the equator.
Between these permanent anticyclonic belts there is a belt of low pressure
which is relatively uniform and is known as the DOLDRUMS.
Anticyclones are also formed over continental areas in winter time, a
noteworthy one being that over Siberia. In the Siberian anticyclone of
winter the pressure is often as high as 1050 mb. or 31 -00 in., while in another
marked anticyclone that occupying the North Atlantic in the region of the
Azores pressure is usually about 1025 mb. or 30-25 in. in the summer and
1020 mb. or 30-10 in. in winter. During an anticyclone the pressure is as a
rule well above the normal for any given place in it and as a consequence
pressures of 1020 mb. (30 10 in.) or over are frequent and such high pressures
are usually taken alone as a sign of the presence of an anticyclone.
In a well-defined anticyclone the winds blow spirally outwards in
accordance with BUYS BALLOT'S LAW. Near the centre, however, the winds
are generally light and very variable in direction, calms being frequent.
In the outer regions, away from the centre, the winds increase somewhat,
arrange themselves in a clockwise direction, move outwards from the area
of high pressure and generally cut the isobars at a small angle. It follows
that, in the northern hemisphere, the winds on the northern side of an anticyclone are westerly and on the southern side easterly.
Anticyclonic weather is usually, and rightly, regarded as quiet and settled,
but in practice considerable variations of weather may be experienced. In
summer over the British Isles the weather in an anticyclone is generally fine,
sunny and warm but much cloud may occur with some rain in its outer portions.
On the other hand in winter there are two distinct types, in one the sky is
covered with a layer of STRATOCUMULUS cloud, conditions which are sometimes
referred to as " anticyclonic gloom," and in the other the sky is almost
cloudlesst, the nights are frosty and, in the early morning, fog is thick and
often widespread. In the former type the cloud height is often uniform over
a large area. The cloud sheet is frequently thin and above it the sky is cloudless.
Temperature observations taken in the upper air show that there is a
temperature inversion or rise of temperature with height just above the
cloud. When in the London area in winter the cloud height is low the lower
surface of the cloud becomes inky black ; the inversion of temperature
coupled with the light winds prevents the escape of London's smoke ; the
light from the sun is almost entirely cut off and the day becomes like night.
Anticyclones usually cover a larger area than depressions and move much
more slowly. When once formed they often remain almost stationary for two
or three days and sometimes for ten days or more. Their general direction
of movement is from west to east. In an anticyclone the upper air layers very
gradually sink and become heated by ADIABATIC compression. This results in
the inversion of temperature to which reference has already been made.
Sir Napier Shaw has estimated that in a large anticyclone the velocity
of the descending air currents is of the order of 300 ft, per day and in small
anticyclones the velocity may be as great as 1,000 to 1,500 ft. per day. These
velocities may be exceeded in the developing stage. (See SUBSIDENCE.) When
the upper air temperatures in an anticyclone are low, that is, when the anticyclone is of little vertical depth, it usually travels more quickly than one
which is of greater depth and warm in its upper layers.
ANTI-TRADES
16
There have been many theories as to the cause and method of formation
of anticyclones but as yet there is no single satisfactory explanation.
Temperature observations in the upper air, however, have shown that between
the heights of two and ten kilometres (one and six miles) the temperature in
an anticyclone is usually higher, and above ten kilometres lower, than the
average for locality and season.
Anti-Trades. At a height of 2,000 metres or more above the surface in
trade-wind regions the wind direction is sometimes reversed, giving for
example a SW. wind on the Peak of Teneriffe. These winds are believed to
be the return currents carrying the air of the TRADE WINDS back to higher
latitudes, hence they are termed " anti-trades " or counter-trades, but they
are not regularly developed. See also ATMOSPHERIC CIRCULATION.
Anti-Twilight. See PURPLE LIGHT.
Anvil Cloud. Cloud having a projecting point or wedge like an anvil.
The form is usually assumed by the tops of fully developed CUMULONIMBUS
clouds, causing heavy showers or thunderstorms. A small anvil is seen in
Fig. 13. The anvil normally consists of ice crystals or snow flakes, and is
fibrous or nebulous in appearance. The term is sometimes used for a large
mass of cirriform cloud on the top of cumulonimbus, even when the lateral
extension is slight. See CLOUDS.
Aqueous Vapour. Aqueous vapour is always present in the atmosphere,
but it never represents more than a small fraction of the whole. In spite
of this fact its presence is of great importance on account of its physical
properties.
On account of its ready absorption of long-wave RADIATION, such as is
given out by objects at ordinary atmospheric temperatures,* it exercises a
strong control over the fall of temperature on clear nights ; when present
in large quantities it checks the loss of radiation from the ground and so
prevents the surface air from being chilled to the extent that it would otherwise have been. Since, on the other hand, its absorption of the visible
radiation of the sun is relatively small, it can to some extent act as a " trap "
for the solar radiation. Even more important thermal effects arise from the
fact that when water vapour is formed as a result of evaporation much heat
is absorbed 597 CALORIES for each gram of water evaporated at 273°A and
an equal quantity is given out when condensation occurs. Now a large
proportion of the evaporation from the whole earth takes place in the tropics,
where most solar radiation reaches the surface; much of the water vapour
so formed finds its way into middle and high latitudes and supplies most of
the moisture for the clouds and rain associated with cyclonic depressions.
In this process, heat is conveyed from low to high latitudes and the climates
of both regions are rendered less extreme. The energy of many meteorological
henomena from the ordinary cumulus cloud to the thunderstorm, and
probably of the tropical cyclone also, is derived from the sun only after first
being stored as latent heat in water vapour ; the latent heat set free during
condensation tends to make the air where condensation is going on warmer
than its environment, and so favours CONVECTION.
If air at a given temperature is confined in contact with pure water,
some of the water will, in general, evaporate and the partial pressure of the
water vapour in the air will increase. At any given temperature, however,
there is a definite upper limit to the amount of water that can be taken up
in this way. When this limit is reached the air is said to be saturated, and the
partial pressure of the water vapour (commonly called the vapour pressure)
is then equal to the saturation vapour pressure for that temperature. It
will be seen from the table below that the saturation vapour pressure increases
rapidly with the temperature.
Air is unsaturated if the actual partial pressure of the water vapour
present is less than the saturation vapour pressure for the temperature of
*Spectrum analysis shows that it absorbs also a small part of the visible radiation of the
sun.
ARID
17
the air. In these circumstances, the hygrometric state of the air may be
expressed in the various ways implied by the following definitions :
1. The " saturation deficit " is the difference between the actual vapour
pressure and the saturation vapour pressure.
2. The " relative humidity " is the percentage degree of saturation, that
is to say, the quotient of the actual vapour pressure divided by the saturation
vapour pressure, multiplied by one hundred.
3. The " dew point " is the temperature for which the actual vapour
pressure is equal to the saturation vapour pressure.
4. The " va,pour density " or " absolute humidity " is the density of
the water vapour present in the air, i.e. the weight per unit volume of air.
5. HUMIDITY-MIXING-RATIO (q.v.).
Averages of the vapour pressure, relative humidity and vapour density
for a large selection of stations are given in "Averages of Humidity for the
British Isles", where charts of the monthly and annual distribution
will also be found. Graphs of the seasonal and diurnal fluctuations are given
in E. G. Bilham's " Climate of the British Isles " (London, Macmillan, 1938).
Data for foreign stations are given in Shaw's " Manual of Meteorology "
Vol. II (Cambridge University Press, 1936). Vapour density is termed
" moisture content" in " Averages of humidity for the British Isles " and in
" Climate of the British Isles."
VALUES OF THE SATURATION VAPOUR PRESSURE AND VAPOUR DENSITY
Temperature
°F.
5
10
15
20
25
30
35
40
45
50
55
Vapour
pressure
mb.
1-93*
2-42*
3-01*
3 73*
4-59*
5-63*
6-89
8-40
10-18
12-28
14-76
Vapour
density
gm./m3.
1-62*
2-01*
2-48*
3-04*
3-70*
4-49*
5-44
6-56
7-87
9-41
11-19
Temperature
Vapour
pressure
Vapour
density
°F.
60
65
70
75
80
85
90
95
100
105
110
mb.
17-68
21-08
25-05
29-65
34-98
41-12
48-17
56-25
65-49
76-02
87-96
gm./m3.
13-26
15-70
18-47
21-64
25-29
29-48
34-20
39-60
45-66
52-53
60-26
* Over supercooled water ; the values over ice are lower (see "Averages of Humidity
for the British Isles " p. 28).
Arctic Air. Air arriving directly from arctic regions. The most notable outbreaks of arctic air occur in winter and spring, and they bring snow to those
regions where the topography favours precipitation with northerly winds, and
occasionally to most of the country. It is really a variety of POLAR AIR,
and there is no sharp line of demarcation, since after a sufficient degree of
warming over the sea, the arctic air becomes maritime polar air. See
AIR MASS.
Arctic Sea Smoke. A kind of FOG formed when the air is much colder
than the sea surface. It is necessarily shallow. It forms over inlets of the
sea in arctic waters when very cold air from the land moves out over the sea
or when the opening of a rift in a solidly frozen sea surface exposes water to
air cooled to a very low temperature by radiation.
Arid. A climate in which the rainfall is insufficient to support vegetation
is termed arid. W. Koppen and R. Geiger in their " Klimakarte der Erde "*
use the following formulae for the limits of rainfall for arid and semi-arid
climates :
R = 2t
..
..
..
Rainfall mainly in cold season ..
R = 2t + 14
..
Rainfall evenly distributed throughout the year
R = 2t + 28
..
..
..
Rainfall mainly in hot season ..
* Gotta, Justus Perthes, 1928.
ATLAS, CLIMATOLOGICAL
18
where / is the mean annual temperature in ° C. If the annual rainfall in centimetres is less than R and greater than %R the climate is steppe or semi-arid ;
if it is less than %R the climate is desert or arid.
Atlas, Climatological. An atlas showing the geographical distribution of
some element or elements of climate. Temperature, rainfall and sunshine
are most frequently shown, but other elements such as wind velocity and
direction, cloudiness, humidity, etc., may be included.
The most usual method of representation is by means of isograms and
sometimes in order to aid the eye the areas between consecutive isograms are
shown in different colours. Isograms may be drawn to show the mean value
of the particular element for any space of time, but the time units most
frequently employed in a climatological atlas are the month and the year.
Many meteorological services have published a climatological atlas for
their country. An atlas for the British Isles is included in Section II of the
" Book of Normals of Meteorological Elements for the British Isles," giving
isograms of temperature, rainfall and sunshine. Numerous charts are also
included in the " Manual of Meteorology," Vol. II, by Sir Napier Shaw.
(Cambridge University Press, second edition, 1936.)
Atmosphere. A name given to the air which surrounds us, and is carried
along with the earth. It is a mixture of gases, and at levels accessible to
measurement, the relative proportions of the permanent constituents show
no appreciable variations, with the sole exception of water vapour. Local
sources of pollution such as factory chimneys may give rise to enhanced
proportions of non-permanent constituents, but these slight local variations
are not of meteorological importance. It is therefore convenient in
meteorology to regard the atmosphere as made up of a homogeneous mixture
called " dry air," whose constitution is everywhere the same at least up to
heights accessible to direct observation, to which is added water vapour in
varying amounts.
The constitution of the atmosphere at heights above about 30 Km. is
uncertain. Theoretical discussion of the change in constitution with height
is based upon the assumption of Dalton's Law, which states that the distribution
of each element at the higher levels is independent of all the other elements
present. The partial pressure of any particular element at, say, 30 Km.
must, therefore, support all the mass of that element above 30 Km. In the
practical application of this rule, considerable importance attaches to whether
hydrogen is regarded as a normal constituent of the atmosphere. In any
case theory demands that nitrogen and oxygen should cease to account for
an appreciable fraction of the atmosphere at heights of 160 Km. and above.
According to Dobson, however, the observation of meteors indicates that up
to the greatest heights at which meteors appear, about 160 Km., oxygen and
nitrogen remain the chief constituents of the atmosphere.
The presence of OZONE in the atmosphere has been demonstrated
spectroscopically by Fowler and Strutt, and observations of absorption of
solar radiation in a restricted band of wave-lengths, by Dobson and others,
confirm this, and indicate that the greater part of the ozone occurs in a layer
20 to 40 Km. above the earth's surface. The presence of ozone at these levels
appears to be of some meteorological significance, Dobson having found that
the pressure at 10 Km. is highly correlated with the amount of ozone.
The variability of water vapour is of the greatest importance in
meteorology. The processes of evaporation and condensation of water vapour
are among the most potent meteorological factors, not only because they
produce a transport of water from one place to another but also because
the evaporation of water to form water vapour requires the absorption of
latent heat, which is finally liberated and used in heating the air when the
water vapour again condenses into waterdrops or snow. The total amount
of water vapour which the air could hold if saturated, under summer conditions
for the British Isles, would yield 35 mm. of rain if it was all precipitated.
The corresponding figure for winter is about 15 mm.
Nitrogen, which is a constituent of ammonia, nitric acid and the nitrates
which are so important in gunpowder and nearly all other explosives, is
quite inert in the atmosphere. It m erely dilutes the more important constituent
19
ATMOSPHERIC CIRCULATION
oxygen, which forms about one fifth of the atmosphere. Without oxygen no
fire can be maintained and the chemical processes in the body necessary for
life cannot go on. But these characteristics of oxygen are of no special
importance in meteorology.
The characteristics of the atmosphere which are of importance in
meteorology are its pressure, temperature, and humidity. To a high degree
of approximation the pressure of the atmosphere at any point is a measure
of the mass of a cylinder of air of unit cross section extending to the top of the
atmosphere. Increase or decrease of pressure therefore connotes respectively
transport of air to, or from, the place of observation. The pressure of the
atmosphere at sea level is equal to a pressure of about 14£ Ibs. per sq. in.
The homogeneous atmosphere is the atmosphere which retains throughout
the whole of its height the density at sea level, and yields normal atmospheric
pressure at the ground. Its height is approximately 8 Km.
See also AIR, STANDARD ATMOSPHERE, STRATOSPHERE.
Atmospheric Circulation. Used in the sense of " general circulation of the
atmosphere " for the wind system of the earth as a whole. This wind system
is related to the pressure distribution over the earth, and both depend ultimately
on the sun's heat. (The relation of wind to pressure distribution is described
under BUYS BALLOT'S LAW.)
Examination of a series of synoptic maps of the hemispheres would show
that certain features of the pressure and wind distribution over the world
are much more variable from day to day than are others. In the northern
parts of the British Isles, for example, pressure not infrequently varies by as
much as 20 mb. in a day, and a day of strong south-easterly winds may be
succeeded by a day of north-westerly gales, while over parts of the South
Atlantic a fresh SE. wind blows with great regularity day after day. If,
however, a series of charts is constructed from the prevailing winds for
periods of a month, a greater degree of regularity appears, and in order to
obtain a picture of the general circulation it is necessary to study such maps
as those of Figs. 3 and 4. These maps represent the stream-lines in January
and July ; the course of the seasons may be considered as a change from
one type to the other.
It is generally agreed that the " planetary circulation," appropriate to a
planet like the earth but of uniform surface, would comprise, at the surface
(a) A belt of low pressure, calms or of light variable winds, with converging air on the equator (DOLDRUMS).
(b) Belts of TRADE WINDS between the doldrums and lat. 30° N. and
30° S. blowing from the north-east in the northern and from the southeast in the southern hemisphere.
(c) Belts of light variable winds, with diverging air which descends
from higher levels in about lat. 30° N. and 30° S. (HORSE LATITUDES).
(d) Regions of prevailing south-westerly winds in middle latitudes of
the northern hemisphere and north-westerly winds in middle latitudes
of the southern hemisphere.
(e) Regions round the poles of outflowing winds with a component
from E.
The transitions between (d) and (e) are regions of variable winds forming the
" temperate storm belts."
Over the oceans (except the Indian Ocean in July) an approximation to
the planetary circulation is observed. The maps show centres of divergence
in about latitude 30°, on the equatorial side of which the trade winds converge
towards a line (the " intertropical front ") a little north of the equator,
while the great south-westerly currents in the northern hemisphere and northwesterly currents in the southern hemisphere are also clearly marked. In
January in the northern hemisphere there are centres of convergence at the
Aleutian and Icelandic lows but in July these are less clearly marked, and
take the form of a front (the POLAR FRONT). In the southern hemisphere
there is a great system of westerly winds in about 40° S, known as the
ROARING FORTIES.
ATMOSPHERIC CIRCULATION
20
Over the continents (and the Indian Ocean) there is a much greater
annual variation. In January a wind axis extends from west-south-west to
east-north-east across Europe and Asia, separating the south-westerly winds
blowing towards the Arctic from northerly winds which give rise to the
HARMATTAN of north Africa and the winter monsoon of Asia. Africa is
21
ATMOSPHERIC CIRCULATION
occupied by four great systems of winds, blowing from the Mediterranean,
south-west Asia, the south Indian Ocean and the south-east Atlantic. Most
.a
1
of North America shows a fairly simple outflow of polar air while the greater
part of South America is occupied by air from the Atlantic. The main
"fronts" between the various air masses are shown by broken lines.
ATMOSPHERIC ELECTRICITY
22
In July southern and eastern Asia are occupied by tropical monsoon air,
but most of Asia and the whole of Europe come under the influence of temperate W. or NW. winds. North America has Arctic air in the north, Pacific
air in the west and moist tropical Atlantic air from the Gulf of Mexico in
the south-east. Both in January and July the cold plateau of Greenland
shows a system of outflowing winds ; a similar outflow probably exists all
round the Antarctic continent.
It is not yet possible to describe the circulation in the upper air in detail,
but investigations in recent years have made known the main lines. Owing
to the decrease of temperature in the troposphere from the equator to the
poles, pressure decreases upwards more rapidly in high than in low latitudes.
Hence the westerly winds of temperate latitudes tend to become stronger and
more regular with increasing height, while the easterly winds of low latitudes
tend to be replaced by westerlies. On a very broad generalised view, in the
northern hemisphere the boundary between the winds with an easterly and
those with a westerly component, which at sea level in winter lies in about
latitude 25° N. is found about 15° N. at a height of 5 Km. and 7° N. at
8 Km., giving a fairly uniform slope upwards towards the equator of about
one in 250. In summer the boundary is shifted northward by about ten
degrees and the slope apparently becomes steeper, as would be expected from
the smaller horizontal temperature gradient at this season. In the southern
hemisphere conditions appear to be similar.
At high levels in the stratosphere the horizontal gradient of temperature is reversed, the air at about 13 Km. being coldest over the equator and
warmest over the poles. Hence above this level there is a tendency for the
westerly component of the winds to decrease again, and the movements of
the Krakatao volcanic dust showed that at heights of 27-33 Km. the winds
were easterly from about 25° N. to 25° S., reaching a speed of nearly 80 m.p.h.
near the equator, but beyond 30° N. and S. the winds at this height were
still westerly.
Atmospheric Electricity. The various electrical phenomena occurring
naturally in the atmosphere. Of these phenomena the most familiar is the
THUNDERSTORM, but there are manifestations which can be studied at all
times with suitable apparatus.
Potential gradient.—It is well known that the wire attached to a kite
sent up to a considerable height, even in fine weather, becomes electrified
so that sparks may be drawn from the wire. This is evidence that there is
a large difference of potential between the ground and the air at the level
reached by the kite.
The measurement of potential at a moderate height is made most readily
by the use of an insulated wire carrying a burning fuse and connected to a
suitable electrometer. As long as the fuse is not at the potential appropriate
to its position a charge will be induced on it. This charge is dissipated by
the burning fuse until the potential has been equalised.
It is found that in the open, potential changes fairly uniformly with
height; the rate of change is called the potential gradient. Near the ground
in fine weather the potential gradient is of the order 150 volts per metre.
In such weather the higher potential is at the greater height and the potential
gradient is said to be positive. The gradient fluctuates continually and also
shows regular annual and diurnal changes. At many places the diurnal
change is such that the gradient has two maxima and two minima in the
course of the 24 hours. For example, at Kew Observatory in July, the
average value (1905-12) of the fine weather gradient at 3h. is 153 v./m.;
there is a rise to the maximum, 272 v./m., which occurs at 9h. The gradient
falls steadily until it reaches a value of 179 v./m. at 14h. and rises again to
266 v./m. at 21h.
The gradient depends to a great extent on the condition of the atmosphere.
It is low when the air is clear, higher when there is haze and very high when
fog prevails. In fog the gradient at Kew may be as high as 2,000 v./m.
23
ATMOSPHERIC ELECTRICITY
Over the oceans and in polar regions the diurnal variation of potential
gradient is governed by world-time, the minimum occurring about 3h. G.M.T.,
the maximum about 19h. G.M.T.
During rainfall and especially during thunderstorms potential gradient is
subject to great fluctuations. In thunderstorms the gradient is of the order
10,000 v./m. positive or negative.
Observations from balloons indicate that on the average as we go up
from the earth the potential gradient diminishes. As, however, the contributions are in the same direction, the potential goes on mounting and at
the levels attained by aircraft it may reach hundreds of thousands of volts.
(In the case of a kite-balloon tethered to the ground by conducting wires
the potential of the balloon is practically zero and, therefore, departs widely
from that of the surrounding air. In an extreme case this may lead to
discharge by sparking and great danger to the kite-balloon.)
The earth charge. —The existence of a positive potential gradient implies
the presence of a negative charge on the ground. With a gradient 100 v./m.
the charge is roughly 1 coulomb per 1,000 square kilometres (accurately
coulomb per cm. 2). If the same gradient prevailed over the whole
globe the total charge would be 450,000 coulombs. To the electrical engineer
this is a very small quantity of electricity. It could only supply a current
of one ampere for five days. Of course the gradient is not uniform ; at any
time there are large areas with negative gradient. We cannot give the average
gradient and we are, therefore, unable to determine the total charge.
Air-earth current. —The charge on the earth's surface is continually being
neutralised by the communication of positive electricity from the atmosphere.
By substituting a metal plate for part of the ground we can measure the
current. The average current over flat ground is of the order 2 microamperes
per square kilometre. Small as this current is it would suffice to neutralise
the fine-weather charge on the ground in a fraction of an hour. There is, of
course, a reverse current at such times as the potential gradient is negative.
Electricity is also conveyed to the ground by rain and by lightning. The
question whether these causes suffice to preserve a balance between the flow
of positive and of negative electricity is still open.
Electricity on rain. It is found that raindrops are usually electrified.
The charges may be positive or negative but positive ones predominate.
Drops with charges of either sign may fall simultaneously. The charge per
cubic centimetre of rain is generally less than one electrostatic unit (£ x 10~ 9
coulomb) but charges approaching 20 e.s.u. per cm. 3 have been observed.
The charges on snow are of the same order of magnitude.
The principal cause of the electrification of rain is probably the breaking
up of the drops.* When the velocity with which a large drop is falling through
the air exceeds a certain limit the drop is broken up into a number of small
drops which become positively charged in the process. According to some
investigators the corresponding negative charges are carried away by the
finer spray, according to others by the air. The process will be accelerated
where there are powerful upward currents of air to break up the drops. Similar
effects will occur with snow. Sir George Simpson based a theory of thunderstorms on these phenomena. In the parts of an active cumulonimbus cloud
where the upward currents of air are most vigorous large drops are formed
by the condensation of water vapour and by the amalgamation of small
drops. In the strong currents the large drops are broken up and become
positively electrified. The air with its negative charge passes on to other
parts of the cloud. Large quantities of electricity, positive and negative,
accumulate in different regions and the electric forces increase until a lightning
flash occurs. The process is repeated as long as the storm lasts.
/\
* " On the electricity of rain and its origin in thunderstorms," by G. C. Simpson, London
Philos. Trans., A. 209, 1909, pp. 379-413.
ATMOSPHERIC POLLUTION
24
Soundings with the altielectrograph an instrument designed to record the
sign of the potential gradient aloft have indicated, however, that very
frequently the charges in the upper parts of thunder clouds are positive.
The temperature at such heights is well below the freezing point and it
appears that some process in which snow plays an important part is
responsible for the greater part of the electrification. It may be that the
impact of snow particles on one another suffices, the particles acquiring
negative charges which they carry downwards while the air, streaming
upwards, retains a positive charge.
lonization of the atmosphere.—The conductivity of the air is due to the
presence of charged particles known as ions. The ions which are effective
in carrying the current are probably clusters of molecules of the atmospheric
gases. On the average there are five or six hundred of these small ions of
each sign in a cubic centimetre of air, each small ion carrying the electronic
charge. The mobility of the small ions is such that they move about one
centimetre per second in an electric field of 1 volt per centimetre. The
atmosphere contains also more numerous ions (the Langevin ions) with much
less mobility. It is thought that these sluggish ions are charges attached to
Aitken nuclei, the hygroscopic particles on which moisture condenses when
clouds are formed. The fact that in general potential gradient falls off as
we ascend in the atmosphere is evidence that the net charge on the air is
positive. This implies that there must be an excess of positive ions.
The variations in the conductivity of the air are explained by the different
proportions of large and small ions. In places where the atmosphere is
polluted by the products of combustion of coal Aitken nuclei are frequent,
large ions are formed at the expense of small ones, the effective conductivity
is reduced and the air-earth current is maintained by a higher potential
gradient.
At appreciable heights above ground the conductivity of the air is
considerable. At 9 Km. Wigand found the conductivity to be 26 times as
great as near the ground. Gish and Sherman found that the increase from the
ground up to 18 Km. was about one hundredfold. The great conductivity of
the Heaviside layer is due to high ionization.
Since free ions of opposite sign will tend to recombine and give up their
charges there must be active causes of ionization at work. Of known causes
the radiation from radio-active gases and the COSMIC RADIATION from outer
space may be mentioned. Combustion and similar processes are also effective
and under certain circumstances the breaking of waterdrops and the friction
of snow or dust driven by* the wind. Over the land most of these causes are
in operation, over the sea the cosmic radiation is the only one which is known
to be effective, but this may suffice to maintain the observed ionization.
Atmospheric Pollution. The principal source of the pollution of the
atmosphere is the burning of coal under such conditions that the combustion
is incomplete. When a hydrocarbon, such as one of the constituents of
coal, is burned, the hydrogen combines with oxygen to form water vapour,
the carbon combines with oxygen to form carbon dioxide. If, however, the
temperature is not high enough the second half of the process does not take
place ; some of the carbon is carried up with the rising air and coagulates
into particles of soot. The smoke from a coal fire consists in the main of such
particles.
The pollution of the air of cities has been reduced by the introduction for
industrial processes of more efficient furnaces in which the coal is completely
burned and very little smoke is produced. Further great quantities of coal
are treated in gas works. By heating the coal in retorts the hydrocarbons are
broken up ; the hydrogen with other constituents of coal gas is distributed
through mains and the coke, which is mostly carbon, is sold separately.
Both the coal gas and the coke will burn without the production of smoke.
The substitution of the gas-cooker for the kitchen range has led to a great
reduction in the amount of domestic smoke.
25
ATMOSPHERIC PRESSURE
The gaseous products of combustion are mostly water vapour and carbon
dioxide, both normal constituents of the atmosphere ; but coal generally
contains sulphur and the burning of sulphur produces the essential constituents
of sulphureous and sulphuric acids. Most of the damage done to buildings
by atmospheric pollution is attributed to the action of sulphuric acid.
A Committee of Investigation having been appointed at a Conference
held in London in 1912 systematic records of atmospheric pollution have been
kept in Great Britain since 1914.
The estimation of the quality and amount of polluting substances in the
air at any time presents great difficulties. It is easier and no less instructive
to measure the pollution which is deposited. For this purpose a large gauge
is used in which rain is collected and also the foreign matter which falls as
dust or is brought down with the rain. Month by month the contents of the
gauge are taken away for analysis.
As an example of the results the 5-year averages for a gauge in the City of
London may be quoted. The quantity of matter deposited in the gauge in
the solid form was 82 grams per square metre per annum. Of this total only
32 gm. was carbonaceous insoluble matter; as such things as pollen would
be included in this 32 gm. the amount of soot would be rather less than 32 gm.
The amount of insoluble ash, i.e. dust from the roads as well as from fires
was 50 gm. The solid matter in solution in the rain water was nearly equal
to the insoluble matter. The soluble matter included a high proportion of
sulphates ; it is the practice of chemists in such cases to estimate the acid
without regard to the base with which it may be associated. In this case the
average amount of SO3 was 25 gm. It will be seen that the quantity of SO3
is not much less than the quantity of soot.
It is found that in places with less pollution than London the ratio of the
amount of soluble matter to the amount of insoluble is higher.
At seaside places such as Southport a considerable amount of sea salt is
included in the soluble matter.
For estimating the amount of solid matter in suspension in the atmosphere
it is found convenient to aspirate a known quantity of air through filter
paper and to compare with a set of standard shades the stain left on the
filter paper. Dr. J. S. Owens has devised automatic apparatus for making
records which are practically continuous.
It is found that in foggy weather the quantity of suspended solid matter
in London air may amount to four milligrams per cubic metre. Generally
the maximum on a foggy day is about 2 mg./m. 3 whilst the maximum on a
non-foggy day in winter is about 0 8 mg./m. 3
Pollution varies systematically through the 24 hours ; normally the least
pollution is in the early morning, when few fires are in operation. There is
also a secondary minimum in the middle of the day at the time when the
air currents are strongest. The times of maximum pollution are generally
about 9 a.m. and 7 p.m.
Dr. Owens has also developed a method of counting the number of solid
particles in the air. For this purpose a jet of air is pumped through a slit
on to a piece of glass which can be examined under the miscroscope. The
number of particles in fog may be as high as 20 per cubic millimetre. The
particles are generally ultra-microscopic.
Atmospheric Pressure. The pressure of the atmosphere is produced by
the weight of the overlying air. If a vertical tube were placed over one
square inch of the earth's surface extending to the top of the atmosphere,
the air contained within it would weigh about 14 J Ibs. As the weight of ithe
column is borne by the earth's surface at the bottom, the pressure exerted
on the earth by the air is therefore about 14£ Ibs. to the square inch. The
pressure exerted by a gas is the same in all directions, and it is this pressure
of approximately 14£ Ibs. to the square inch but varying a little from day to
day and from place to place which is measured by the mercurial or aneroid
barometers in ordinary use.
ATMOSPHERICS
26
Atmospherics. Electrical impulses of natural origin which cause crashing
or grinding noises in a wireless receiving set. They are believed to originate
in the earth's atmosphere, most probably in discharges of the same nature
as lightning. Atmospherics of measurable strength are received in England
at the rate of about 50 per second at night, and in the tropics about 50 per
second during the day and a few hundred, rising occasionally to a few thousand,
per second, during the night. By means of radio direction recorders, the
sources of many atmospherics have been determined, and it has been found
that some radiate from centres 4,000 miles away. The average strength of
atmospherics received in England is consistent with their radiation from
centres 2,000 miles away, which in turn is consistent with their origin in regions
in which thunderstorms are frequent.
Attached Thermometer. A thermometer attached to a mercurial barometer
for the purpose of ascertaining its temperature. When used in conjunction
with a marine barometer the attached thermometer may conveniently be
mounted in a GOLD SLIDE.
Audibility. The audibility of a sound in the atmosphere is measured by
the distance from its source at which it becomes inaudible. On a perfectly
clear, calm day the sound of a man's voice may be heard for several miles,
provided there are no obstructions between the source of sound and the
listener ; but quite a small amount of wind will cut down the range of audibility
enormously.
The sound is not cut down equally in all directions ; to leeward, for
instance, a sound can usually be heard at a greater distance than it can to
windward of the source. This is accounted for by the bending which the
sound rays undergo, owing to the increase in wind velocity with height above
the ground, the rays to leeward of the source being bent downwards while
those to windward are bent upwards so that they pass over the head of an
observer stationed on the ground. The decrease in all directions in the
range of audibility of a sound when there is a wind, appears to be due chiefly
to the dissipation in the energy of sound as it passes through eddying air.
A plane wave-front becomes bent in an irregular manner when it passes
through air in irregular or eddying movement. It, therefore, ceases to travel
uniformly forward. Part of its energy is carried forward, while the rest is
dissipated laterally.
Another process must also be taken into account. It has been found by
V. O. Knudsen that, when a noise is produced in a small room the rate at
which the reverberation dies out depends on the humidity of the air, and
further that when oxygen is substituted for air the damping is more rapid
whereas when nitrogen is substituted there is practically no damping. An
explanation of the phenomena has been given by H. O. Kneser. Molecules of
oxygen are liable to pulsations in which the distance between the nuclei of
the constituent atoms is variable. The pulsations are produced by encounters
between the oxygen molecules and molecules of water vapour, but the
pulsations are not very readily started and when they do start they are
not readily stopped. This sluggish transfer of motion robs a sound wave
passing through the atmosphere of part of its energy. How effective the
Knudsen-Kneser phenomenon may be in producing the absorption of sound
in the open air is not known yet, i.e. at the end of 1938.
If there were no dissipation of energy in a sound wave the intensity of
the sound would decrease inversely as the square of the distance from the
source. Experiments show that, under normal conditions when there is a
light wind blowing, the rate of decrease in intensity of sound at a distance of
half a mile or more is considerably greater owing to the dissipation of energy
than would be expected from the inverse square law. If, for instance, a
whistle can be heard at a distance of half a mile, four whistles blown
simultaneously should be audible at a distance of a mile ; but the range is
actually only increased to about three quarters of a mile.
27
AUDIBILITY
Sounds are usually heard at greater distances during the night than
during the day. On calm nights the range of audibility of a sound may be
as much as 10 or 20 times as great as it is during the day. This effect is due
partly to the increased sensitiveness of the ear at night owing to the decrease
in the amount of accidental disturbing sounds, partly to the inversion of
temperature which commonly occurs on calm, clear nights, and has the
effect of bending the sound waves downwards, but chiefly to the diminution
of the amount of disturbance in the atmosphere at night.
Between the source of sound and the extreme range of audibility, areas
of silence sometimes appear, in which the sound cannot be heard. This
(Reproduced by permission from the Quarterly Review, July, 1917.)
FIG. 5. Areas of audibility of the explosion at Silvertown on January 19, 1917.
effect has in some cases been attributed to a reversal in the direction of the
wind in the upper layers of the atmosphere. The lower wind would bend the
sound rays upwards to windward of the source. On entering the reversed
upper wind current these rays would be bent down to the earth again, and
would reach it at a point separated from the source of sound by an area of
silence. This explanation is quite adequate in many cases in which the
places, where the sound is heard again, are to the windward of the source.
There are, however, many cases in which areas of silence appear to leeward
of the source, and many others in which an area of silence occurs in the form
of a ring enclosing the source and surrounded by an area of distinct audibility.
AUREOLE
28
In most of the cases where a ring-shaped area of silence has been observed the
outer region of distinct audibility begins at a distance of about 100 miles from
the source, and may extend to 150 miles or more. The explosion at Silvertown
on January 19, 1917, is a good example of a case in which a detached area of
audibility was separated from the source of sound by an area of silence. In
the accompanying map (Fig. 5), which is reproduced by permission from the
Quarterly Review, the two areas of audibility are shown. It will be seen that the
outer area, which includes Lincoln, Nottingham and Norwich, lies about 100
miles from the source of sound. The inner area surrounding the source is not
symmetrical, being spread out towards the north-west and south-east.
Definite evidence was obtained that no sound was heard at various points
within the area of silence.
The study of observations made on artificial explosions has shown that
the time which the sound takes to reach a point in the inner zone of audibility
near the source is such that the apparent velocity difiers but little from the
ordinary velocity of sound, but that the apparent velocity with which the
sound reaches a point in the outer zone is much less. For this reason the outer
zone is known as the " zone of abnormal audibility." The probable explanation
of abnormal audibility is the presence of a region of high temperature at a
considerable height above the ground. Owing to the way in which temperature
falls off in the lower atmosphere sound rays are generally bent upwards ;
in the stratosphere in so far as it is a region of uniform temperature the
rays will be straight. For the rays to recurve sufficiently to bring them down
again they must enter a region where the temperature is at least as high as that
which prevails on the ground. The observations indicate that the rays reach
heights of 40 Km. or more. That high temperatures prevail at such levels was
first suggested as an explanation of the behaviour of METEORS. The observations
of audibility have confirmed the theory.
Aureole. A luminous area surrounding a source of light. The name is
used especially with reference to coronae. When there is a corona round the
sun or moon a clear space in which the cloud seems to be transparent is seen
near the luminary. The first coloured ring, usually a brownish-red, bounds
this clear space. The clear space is the aureole. The term may be used
legitimately when the corona is not developed beyond the first coloured ring.
The name aureole is also used by some authors for the bright area with
no definite boundary seen round the sun when no clouds are present.
Aurora. In Latin the word aurora stands for dawn. According to the
Oxford Dictionary the term aurora borealis was introduced by Gassendi in
1621. This term, the northern dawn, is apt enough for a faint glow seen at
night near the northern horizon. The same name is given, however, to a
wider class of luminous phenomena. On the recognition of similar phenomena
in the Antarctic regions, these were called aurora australis. The single word
aurora is now in general use in either hemisphere.
Aurora takes many forms. Of the accompanying figures one shows an
arc with rays, the other a curtain. The quiet arc is the most symmetrical
and stable form of aurora, sometimes persisting with little visible variation
for several hours, but an arc with ray structure such as is shown in the
illustration undergoes rapid changes. The lower edge, both of arcs and curtains,
is usually much the better defined. Another striking form of aurora is that
known as a corona, in which several beams of luminescence radiate from a
point high in the sky. Several forms may occur simultaneously or successively
during any auroral display.
Aurora is very rare in southern Europe, and is but seldom seen in the
south of England. But the frequency increases rapidly as we go north, and
in Orkney and Shetland aurora is a common phenomenon. In those regions
it may occasionally be observed to the south as well as to the north.
A zone of maximum frequency passes across the north of Norway, the
south of Iceland and of Greenland and surrounds the north magnetic pole ;
the distribution in the Antarctic is more or less similar. Cape Evans, the
To jace page 23.
FIG 6.—Aurora of February 19, 1915
22h.
FIG 7 —Aurora of January 4, 1917. Sketched at Aberdeen by G. A Clarke.
Phase I. 20h. 20m.
' '
(90565)
29
AVERAGE
headquarters of Scott's last expedition, was within the auroral zone ; aurora
was seen frequently to east-north-east and comparatively rarely overhead.
It is difficult to assign laws for the frequency of occurrence of aurora as weak
displays are obscured by moonlight or twilight; also, in daylight or on
nights when the sky is heavily clouded observations of aurora are impossible.
In the British Isles and similar northern latitudes it is most common in the late
evenings and near the equinoxes ; but in the north of Norway and Greenland
it appears to be most frequent near mid-winter. Of late years Professor
Stormer has devised a method of photographing aurora, including reference
stars in the photograph, and by means of photographs taken simultaneously
at the two ends of a measured base he has obtained numerous results for the
altitude. Heights exceeding 200 Km. are not unusual, but a great majority
of the heights lie between 90 Km. and 130 Km.
Aurora is undoubtedly an electrical discharge, and we thus infer that at
heights of 100 Km.—at least in high latitudes—the atmosphere must often,
if not always, be a vastly better conductor of electricity than it is near the
ground. Some distinguished travellers have claimed to see aurora come down
between them and distant mountains. Such observations are generally
regarded as illusions. Reports of the audibility of aurora must be placed in
the same category.
When visible irf England aurora is nearly always accompanied by a
magnetic storm, but this is not the case when it is confined to high latitudes.
The spectrum of aurora consists of a number of lines, one of which in
the green is particularly characteristic. This line, the wave length of which
is 5.577A, has been shown by McLennan* and his collaborators at Toronto
to be due to oxygen. The great height to which aurora is visible is evidence
that oxygen extends to 200 Km. or more above the ground.
Aurora, Permanent.—A photograph of the spectrum of the night sky will
always show the green auroral line at 5,577 A. Careful observation will
show that the background of the starlit sky is not always equally bright,
even where there is no interference from mist or haze. Sometimes a faint
illumination of patches of the sky may be seen in situations where there is
no possibility of any source of light in the sky other than auroral. These
patches are sometimes referred to as " earthlight." All this points to the
existence of some form of faint permanent aurora, of variable intensity.
The permanent aurora is considered to be unconnected with the solar
disturbances producing aurorae and magnetic storms. Radio investigation
has shown that an electrically conducting layer exists at a height of about
100 miles above the earth's surface, with other similar layers at higher altitudes.
The electrical conductivity is produced by ionization of the atoms of the
atmosphere by the ultra-violet radiation from the sun. During the night
there is a slow reversal of the process producing a faint emission of light.
Since the emission of ultra-violet radiation by the sun is greatest about
the period of maximum solar activity the permanent aurora should, in general,
be brightest at this time.
Autumn.—Autumn in meteorology comprises (for the northern hemi­
sphere) the months of September, October and November. See SEASONS.
Avalanche Wind.—An avalanche often causes a high wind known as the
avalanche wind or avalanche blast. It sometimes causes destruction at a
considerable distance from the avalanche itself.
Average.—This term originally meant the adjustment between the
interested parties of the charges for loss or damage to a ship or cargo at sea.
It is now used in another sense. By the average of a series of numerical
quantities is understood the result of summing the quantities and dividing
* " The spectrum of the aurora and the constitution of the upper atmosphere," London
Proc. roy. Soc., A, 108, 1925, p. 501.
AZIMUTH
30
the total by their number. The term is thus synonymous with the arithmetic
mean. The average value of any meteorological element for a particular
station is obtained in this way by making use of a long and complete series
of observations of the element. Provided there is no reason to believe that
the climate of the station is changing, this average value can be used as the
NORMAL with which individual values of the element can be compared. In
taking the average of a series of quantities in this way it is necessary that
all the quantities should have equal weight, that is, all should be observed
equally carefully and in an identical manner. If the observations are not all
strictly comparable it may be necessary to give greater stress to those thought
to be the best, a process technically known as WEIGHTING.
Azimuth.—The arc of the horizon intercepted between the vertical plane
passing through the observer and the poles of the earth and the vertical
plane passing through the observer and an object.
In nautical astronomy it is measured in degrees (0-180°) eastward or
westward from the elevated pole, but in meteorology it is given in degrees
(0-360°) clockwise from the true north.
Azores Anticyclone.—Or North Atlantic anticyclone, part of the sub­
tropical, high-pressure belt of the northern hemisphere. See ATMOSPHERIC
CIRCULATION, ANTICYCLONE.
Isobaric maps of normal pressure show in summer an area of high pressure
centred about lat. 35° N., extending across the Atlantic to south-western
Europe and the western Mediterranean. In winter the anticyclone is centred
somewhat further south, about lat. 30° N., and is less intense. See CENTRE
OF ACTION.
Backing.—A wind is said to " back " when it is changing in the opposite
direction to the hands of a watch. Thus the wind backs at a place north of
the centre of a depression travelling eastwards in the northern hemisphere.
The same definition of backing applies to the southern hemisphere but as
wind direction in relation to systems of closed isobars is there reversed, a
backing wind in the southern hemisphere is equivalent to a veering wind in
the northern hemisphere, and conversely.
Baguio.—A local name by which the tropical cyclones experienced in the
Philippine Islands are known. A number of the cyclones or typhoons of the
western Pacific cross the Philippines, and in addition there is a class of cyclone
which is especially associated with these islands, occurring from July to
November.
Ball Lightning.—An occasional incident during thunderstorms is the
appearance of ball lightning. The circumstances vary considerably. The
size of the balls varies greatly, from the size of nuts to spheres a foot or two
in diameter. The most frequent balls are between 10 cm. and 20 cm. in
diameter. Sometimes they occur immediately after a brilliant flash of
ordinary lightning, at other times when there has been no flash at all.
Sometimes only a single ball is seen, at other times the same observer sees
two or three and several are reported in one locality. Sometimes the balls
drift through the air and vanish harmlessly, but in some cases a ball has
exploded on reaching the ground. Usually there is no sign of heating where
a ball has passed, yet there is one case where clean holes were bored through
several window panes, the glass appearing to have been melted. Ball lightning
has been observed in closed rooms, but whether it penetrates the walls or
forms inside is not quite certain. The light is seldom brilliant, though
occasionally observers have been dazzled. Cases are on record where a ball
has broken up into smaller ones, but these are rare. In some cases the balls
develop during very heavy rain ; in other cases when there has been no rain
for several minutes. A ball may last for a few seconds or several minutes
31
BALLOON
Its movement is never fast, generally the speed is comparable with a walking
pace, but it is not clear how the ball lightning is propelled. Probably in most
cases it is carried by air currents. Apparently ball lightning does not occur
when there is much wind.
Pearl-necklace lightning.—Ball lightning has been associated occasionally
with pearl-necklace lightning. This phenomenon is a development of an
ordinary lightning flash. Immediately after the flash a number of bright
lights are seen. These lights are of uniform size and appear like pearls on a
string. They last about a couple of seconds.
Phenomena with some likeness to ball lightning have been produced
artificially by the use of very powerful electric machines. N. Hesehus* used
a transformer giving 10,000 volts and connected one pole to a vessel containing
water, the other to a copper plate 2 to 4 cm. above the water. The discharge
between the plate and the water took the form of flames, now conical, now
spheroidal. The fiery spheroids were very mobile, going from side to side of
the copper plate at a breath. In these flames atmospheric nitrogen was
being burned to nitric oxide, NO, as in one of the industrial processes for
" fixing " nitrogen, but the flames did not float away into the free atmosphere.
There is therefore no reason to assume that ball lightning is a globe of nitric
oxide. The fact that the globular form is maintained by ball lightning
seems to require an electrical attraction between the outer layers and a central
nucleus so that the mechanism can not be merely chemical. At present the
phenomenon is entirely inexplicable.
Ballistics.—The science of gunnery. Range tables for ordnance are
constructed on the assumption that certain standard meteorological conditions
are applicable. In actual firings, variations from these conditions will occur
and corrections must be applied when laying the gun, or the shell will not hit
the desired target. The application of meteorology to gunnery came into great
importance during the European War and a specialised branch of the subject
grew out of the requirements of artillery units for upper air data. Methods
for computing EQUIVALENT CONSTANT WIND and BALLISTIC TEMPERATURE
are now in common use at army meteorological stations.
Ballistic Temperature.—A particular type of mean temperature of great
importance in BALLISTICS. A shell in flight meets air of various temperatures
and in computing the ballistic temperature different weights have to be
given to the temperatures experienced, according to the type of shell and the
circumstances of the firing. In any particular caee the effect upon the motion
of the shell of the varying temperature distribution observed is the same as
the effect which would be produced if the temperature at the ground were
equal to the ballistic temperature, and the lapse rate had a certain standard
value.
Balloon.—A bag of impermeable fabric, usually spherical in form, and
inflated with a gas lighter than air. The modern balloon is usually made of
varnished cloth or gold-beater's skin, while over the envelope is a network
of fine cord, from which is suspended the passenger car or basket. At the
top of the envelope is a gas release valve, operated by means of a cord which
hangs down inside the balloon and passes through the neck. A " ripping
panel " is fitted for the release of the gas in emergency. An anchor, guide
rope, and bags filled with sand for ballast are provided. The modern balloon
owes its inception in a practical form to the brothers Montgolfier, who
experimented with hot-air or fire balloons in 1783. Many notable ascents have
been made in free balloons, in connexion with meteorological observations,
sortie of the earliest being made by John Welsh in 1852, and James Glaisher
between 1862 and 1866.
* Phys. Z., Leipzig, 2, 1901, p. 579
BANNER CLOUD
32
Although the spherical balloon may be held captive, at small altitudes
and during light to moderate winds, it is usual to employ the kite-balloon
for this purpose. In connexion with the investigation of the upper air at
considerable altitudes, small rubber balloons are used, having a diameter
of about four feet when filled with hydrogen (see SOUNDING BALLOON).
These balloons carry self-recording instruments, or METEOROGRAPHS. By
this method it is possible to obtain information at far greater altitudes than
can be reached with balloons carrying an observer. For observations of the
direction and velocity of upper winds small rubber balloons are employed,
having a diameter of about 18 inches when filled with hydrogen. They are
liberated from the observing station, and observed, during free ascent, by
means of a theodolite. See PILOT BALLOON.
Larger balloons carrying radar targets are also used and are observed by
radar. By their use the wind may be measured above and in cloud and to
heights of 60,000 ft.
Banner Cloud.—A lenticular cloud that forms in the lee of an obstruction
which causes a forced up-draught of wind, the clear but damp air being then
dynamically cooled by its ascent to the point of cloud formation. Usually
a descent and re-heating of the air will follow at some distance further down
wind, and there the banner terminates. The banner cloud though stationary
itself has a free flow of air constantly through it. The Helm Cloud over
Crossfell Range, the Table Cloth over Table Mountain, Tursui over Mount
Fuji and the cloud over the Rock of Gibraltar during the Levanter, are all
well-known examples of banner cloud.
Bar.—The unit of atmospheric pressure, being equal to the pressure of
one million dynes (one megadyne) per square centimetre. The bar is equal
to the pressure of 29-5306 in. or 750-076 mm. of mercury at 273°A. (32° F.)
and in lat. 45°. The name was introduced into practical meteorology by
V. Bjerknes, and objection has been raised by McAdie, of Harvard College,
on the ground that the name had been previously appropriated by chemists
to the C.G.S. unit of pressure, the dyne per square centimetre. The meteoro­
logical bar is thus one million chemical bars, and what chemists call a bar
we should call a microbar. One bar is 100 centibars or 1,000 MILLIBARS.
Baroclinic.—A term used in dynamical meteorology for the field of mass
when the surfaces of equal density or of equal specific volume cut the isobaric
surfaces in a well-defined system of curves of intersection. A field of mass
may be represented by either of two variables, the density, defined as the
mass per unit volume, or its reciprocal value the specific volume, which is the
volume of unit mass. Both are represented by the same family of equiscalar
surfaces. When the equisubstantial surfaces coincide with the isobaric, i.e.
when the specific volume or density is a function of the pressure, then the
field of mass is barotropic.
Barograph.—A self-recording barometer, an instrument which records
automatically the changes of atmospheric pressure. In one form of mercury
barograph the movements of the mercury in a barometer are communicated
by a float to a pen in contact with a moving sheet of paper carried by a
revolving drum which is driven by clockwork. In another form in use at
Kew and other observatories, the position of the mercury MENISCUS is recorded
photographically.
The portable barographs which are in common use are arranged to record
the variations of pressure shown by an aneroid barometer, and on that
account they are sometimes referred to as ANEROIDOGRAPHS. Particulars as
to the method of using these instruments are given in the " Meteorological
Observer's Handbook."
Barometer.—An instrument for measuring the pressure of the atmosphere.
The mercury barometer has been found to be the most satisfactory form for
general use. The principle underlying this type of instrument is quite simple.
33
BEAUFORT NOTATION
If a glass tube, 3 ft. long, closed at one end, is filled with mercury and the
open end is temporarily stopped up and immersed in an open vessel of the
same liquid, then if the tube is held in a vertical position and the immersed
end is re-opened, the mercury will fall until the level inside the tube stands
at a height of about 30 in. above the mercury in the open vessel. The pressure
of the atmosphere on the lower mercury surface balances the tendency of the
enclosed column to fall, and the height supported in this way represents
the atmospheric pressure at the time. In order to compute the pressure,
the length or height of the column of mercury has to be measured. Different
mercury barometers vary as regards the method of reading this height, and in
all, the temperature of the mercury and the latitude of the place must be
taken into account. Most barometers are now graduated in millibars. See
MILLIBAR, STANDARD TEMPERATURE.
In the " Fortin " barometer, the zero of the scale consists of a fixed
pointer and the cistern is made flexible so that the mercury level in the
cistern may be brought into coincidence with the zero before a reading is
taken. In the " Newman " pattern the cistern is rigid but the scale is mounted
on a movable rod so that a similar zero adjustment may be made by moving
the scale instead of the mercury. In the Kew pattern barometer, no zero
adjustment is provided, but the scale is graduated in such a way that the
changes of level in the cistern are automatically allowed for. Since in such
barometers it is not necessary to see the mercury in the cistern, the latter
may consist of a solid cup, preferably of stainless steel.
In the ANEROID BAROMETER, changes in the pressure of the atmosphere
cause changes in the distance apart of the opposite faces of a closed shallow
metallic box, nearly exhausted of air. These changes are communicated to a
pointer moving over a suitably engraved scale.
For the purposes of meteorology, the pressure of the atmosphere has to
be determined to the ten-thousandth part, which is a much higher degree of
accuracy than is required in other meteorological measurements. Special
contrivances and precautions are therefore required, a description of which
will be found in the " Meteorological Observer's Handbook."
Barometric Tendency.—A term used to denote the change in the barometric
pressure within the three hours preceding an observation. Entered upon
synoptic charts, barometric tendencies are of the greatest value in affording
a ready means of identifying the regions where the barometer is in process
of rising or in process of falling. Lines drawn through places of equal baro­
metric tendency are termed ISALLOBARS and charts of isallobars are useful
for indicating in a clear way the changes in pressure which are taking place.
Barothermograph.—An instrument registering temperature and pressure
simultaneously. The best known form is the Dines balloon meteorograph,
in which a single recording stylus is caused to move in one direction by a
change of pressure and in a direction almost at right angles by a change of
temperature. The trace, therefore, shows corresponding values of temperature
and pressure, from which the height can be computed. The name has also
been applied to instruments in which separate barograph and thermograph
movements are arranged to record on the same chart.
Barotropic.—See BAROCLINIC.
Bearing.—True bearing is the horizontal angle between the direction of
an object and the observer's meridian, measured in degrees (0-360°) clockwise
from the true north. Magnetic bearing is the corresponding angle measured
clockwise from the magnetic north.
Approximate bearings may be named as the compass points, N., NE., etc.
Beaufort Notation.—A code of letters indicating the state of the weather
which was originally introduced by Admiral Sir Francis Beaufort for use at
(90565)
B
BEAUFORT SCALE OF WIND FORCE
34
sea, but which is equally convenient for use on land. Some additions have
been made to the original schedule and it now stands as follows :—
blue sky, whether with clear or
hazy atmosphere.
cloudy, i.e., either detached
c
clouds, or a cloud layer with
openings.
drizzle,
d
wet air without rain falling, a
e
copious deposit of water on
trees, buildings or rigging.
fog.
f
fg ground fog.
shallow fog over the sea.
it
fe wet fog.
gloom.
g
hail.
h
ks drifting snow.
kz duststorra or sandstorm.
lightning.
1
m mist; range of visibility 1,100
yd. or more, but less than
2,200 yd.
b
o
p*
q
r
rs
s
t
tl
u
v
w
x
y
z
overcast, i.e., the whole sky
covered with a uniform
layer of thick or heavy
cloud.
showers.
squalls.
rain.
sleet, i.e., rain and snow to­
gether.
snow.
thunder.
thunderstorm.
ugly, threatening sky.
unusual visibility of distant
objects.
dew.
hoar frost.
dry air (less than 60 per cent,
relative humidity).
haze; range of visibility 1,100
yd. or more, but less than
2,200 yd.
Beaufort used small letters in his notation but under the new convention
a capital letter is used to denote intensity of the phenomenon to be noted ;
at the other end of the scale occasions of slight intensity are indicated by
a small suffix 0 . Continuity is indicated by-a repetition of the letter, intermi ttence by prefixing the letter i and showers by prefixing the letter p. Thus
we have:—
RR continuous heavy rain.
R heavy rain.
rr continuous (moderate) rain.
(moderate) rain.
r
ir0 intermittent slight rain.
rc slight rain,
pS heavy snow showers.
pr rain showers.
For further details in the interpretation of the Beaufort Notation see the
'Meteorological Observer's Handbook."
Beaufort Scale of Wind Force.—Wind force is estimated on a numerical
scale ranging from 0, calm, to 12, hurricane, first adopted by Admiral Beaufort.
The specification of the steps of the scale originally given had reference to a
man-of-war of the period 1800-50 and therefore now possesses little more
than historic interest. Full details of the Beaufort scale, including the original
specification are given in the following table. The introduction of anemo­
meters led to the necessity for a scale of equivalents between Beaufort
numbers estimated by experienced observers and the velocity of the wind
in miles per hour. Experiments in this country showed that the relation
could be expressed approximately by the equation V = 1 • 87 \/ B3 where
V is expressed in miles per hour and B is the corresponding Beaufort number ;
the velocity equivalents in the table are based on this formula. The pressure
equivalents are derived from the formula P = -003 V2 where P is expressed
in pounds per square foot and V in miles per hour.
As the use of the Beaufort numbers spread to other nations it was apparent
that different countries were using different scales of equivalents ; accordingly
* Except for jp, precipitation within sight either not at the station or not reaching the
ground, p is only used as a prefix.
35
BIZE
at the meeting of the International Meteorological Committee in 1921,
Dr. (now Sir George) Simpson was asked to look into the matter of proposing
a definite scale of equivalents. Simpson's proposal as set out in Professional
Notes No. 44, was adopted by the International Meteorological Committee
at its meeting in Vienna in 1926. The following table gives the equivalents
proposed by Simpson for an anemometer at a height of 6 metres above
a level surface.
Limits of velocity
Miles per hour
Metres per second
0- 1
0- 0-5
0
2-3
0-6- 1-7
1
4- 7
1-8- 3-3
2
8-11
3-4- 5-2
3
12-16
5-3- 7-4
4
17-21
7-5- 9-8
5
22-27
9-9-12-4
6
28-33
12-5-15-2
7
34-40
15-3-18-2
8
41-48
18-3-21-5
9
49-56
21-6-25-1
10
57-65
25-2-29-0
11
When an appropriate allowance is made for the height of the anemometer,
these values agree very nearly with those adopted by the Meteorological
Office and given in the table on p. 36.
Code Number
Benard Cell.—A type of EDDY which has been observed in fluids cooled
rapidly at the top in the laboratory and believed to exist also in the atmosphere.
The actual form of the motion in the cell depends upon the variation of
horizontal velocity with height in the fluid.
Bioclimatology.—The study of climate in relation to life and health. With
due attention to housing, clothing, diet and general hygiene nearly the whole
of the earth's surface may be said to be habitable by healthy human beings,
but it is well recognised that in the tropics or in high northern and southern
latitudes conditions are only reasonably tolerable over limited areas, and that
the optimum conditions are found in the temperate zones. In the case of
invalids, particularly those suffering from respiratory or rheumatic diseases,
the permissible variations of temperature and humidity may be relatively
narrow. One of the aims of bioclimatology is to ascertain the climatic con­
ditions most favourable for these and other conditions, and to define the
areas where such climates exist.
Bishop's Ring.—A dull reddish-brown ring which is seen at certain periods
round the sun in a clear sky. In the middle of the day the inner radius of
the ring is about 10°, the outer 20°, but when the sun is low the ring becomes
larger, the brightest part being about 19° from the sun. After sunset the
ring is lost in the warm colours of the sky. Bishop's ring was first observed
after the great eruption of Krakatoa in 1883 and remained visible till the
spring of 1886, and is generally attributed to minute particles shot out by the
eruption and remaining hi the atmosphere at great heights. Bishop's ring
was seen again after the eruptions of the Souffriere in St. Vincent and Mount
Pelee, in Martinique, in 1902. Like the brownish ring surrounding the
AUREOLE observed in clouds the phenomenon is explained by diffraction.
Bize.—A cold, dry wind which blows in the winter in the mountainous
regions of southern France from the N., NE. or NW. The cold NW. wind
which occurs in Languedoc, near the Mediterranean coast, and differs from the
MISTRAL in that it is accompanied by heavy clouds has been given the name
of " bise noire."
(90565)
B2
Calm; smoke rises vertically
..
..
..
..
..
..
Direction of wind shown by smoke drift, but not by wind vanes
..
Wind felt on face ; leaves rustle ; ordinary vane moved by wind
..
Leaves and small twigs in constant motion ; wind extends light flag ..
Raises dust and loose paper ; small branches are moved ..
..
..
Small trees in leaf begin to sway ; crested wavelets form on inland waters
Large branches in motion ; whistling heard in telegraph wires ; umbrellas
used with difficulty ..
..
..
..
..
..
..
..
Whole trees in motion ; inconvenience felt when walking against wind ..
Breaks twigs off trees; generally impedes progress
..
..
..
Slight structural damage occurs (chimney pots and slates removed)
..
Seldom experienced inland; trees uprooted; considerable structural
damage occurs
..
..
..
..
..
..
..
..
Very rarely experienced ; accompanied by widespread damage ..
..
..
..
..
..
..
..
..
..
..
..
..
2-3
3-6
5-4
7-7
10-5
14-0
>17-0
5-0
6-7
>8-l
0
-01
-08
-28
-67
1-31
Lb. per
sq. ft.
1-1
1-7
2-6
3-7
0
-01
-04
-13
-32
-62
mbf
52
60
68
76
85
94
104
114
24
30
37
44
0
2
5
9
13
19
Knots
59
68
78
88
98
109
120
131
42
50
28
35
0
2
5
10
15
21
Miles
per
hour
Equivalent
speed at 33 ft.
48-55
56-63
64-71
72-80
81-89
90-99
100-108
109-118
22-27
28-33
34-40
41-47
1-3
4-6
7-10
11-16
17-21
Knots
55-63
64-72
73-82
83-92
93-103
104-114
115-125
126-136
25-31
32-38
39-46
47-54
1-3
4-7
8-12
13-18
19-24
Miles per
hour
24-5-28-4
28-5-32-6
32-7-36-9
37-0-41-4
41-5-46-1
46-2-50-9
51-0-56-0
56-1-61-2
10-8-13-8
13-9-17-1
17-2-20-7
20-8-24-4
0-0-2
0-3-1-5
1-6-3-3
3-4-5-4
5-5-7-9
8-0-10-7
Metres per
second
At 10m. (33 ft.) in the
open
Limits of speed
* The pressure due to the wind on any object exposed to it arises from the impact of the air on the windward side and suction on the leeward side ; the
mean pressure depends on the shape and size of the object. The values given are for a disc of one square foot in area but they apply with fair approximation
for circular or square plates from 1 sq. ft. to 100 sq. ft. in area.
t One millibar •« 10* dynes per square centimetre — approx. 10 kilograms per square metre.
11
12
13
14
15
16
17
7
8
9
10
0
1
2
3
4
5
6
CO
V
I
Specification of Beaufort Scale for use on land, based on observations
made at land stations
*Mean pressure
(at standard
density) on a
disc of 1 sq. ft.
w
SPECIFICATION OF THE BEAUFORT SCALE WITH PROBABLE EQUIVALENTS OF THE NUMBERS OF THE SCALE >
Light breeze
Gentle breeze
Moderate breeze
Fresh breeze
Strong breeze
Moderate gale* .
Fresh gale
Strong gale
Whole gale
Storm
Hurricane
2
3
4
5
6
7
8
9
10
11
12
Moderate waves, taking a more pronounced long form ; many
white horses are formed. Chance of some spray.
Large waves begin to form ; the white foam crests are more
extensive everywhere. Probably some spray.
Sea heaps up and white foam from breaking waves begins to be
blown in streaks along the direction of the wind.
Moderately high waves of greater length ; edges of crests begin to
break into the spindrift. The foam is blown in well marked
streaks along the direction of the wind.
High waves. Dense streaks of foam along the direction of the
wind. Sea begins to " roll." Spray may affect visibility.
Very high waves with long overhanging crests. The resulting foam,
in great patches, is blown in dense white streaks along the
direction of the wind. On the whole the surface of the sea takes
a white appearance. The rolling of the sea becomes heavy and
shock-like. Visibility affected.
Exceptionally high waves (small and medium-sized ships might be
for a time lost to view behind the waves). The sea is completely
covered with long white patches of foam lying along the direction
of the wind. Everywhere the edges of the wave crests are blown
into froth. Visibility affected.
The air is filled with foam and spray. Sea completely white with
driving spray ; visibility very seriously affected.
Sea like a mirror
Ripples with the appearance of scales are formed, but without
foam crests.
Small wavelets, still short but more pronounced. Crests have a
glassy appearance and do not break.
Large wavelets. Crests being to break. Foam of glassy appear­
ance. Perhaps scattered white horses.
Small waves, becoming longer ; fairly frequent white horses
Specification of the Beaufort Scale for
use at sea (provisional)
Smacks have double reef in mainsail. Care
required when fishing.
Smacks remain in harbour and those at sea
lie-to.
All smacks make for harbour, if near.
Wind fills the sails of smacks which then
travel at about 1-2 miles per hour.
Smacks begin to careen and travel about
3-4 miles per hour.
Good working breeze, smacks carry all
canvas with good list.
Smacks shorten sail.
Calm.
Fishing smackf just has steerage way.
Specification of the Beaufort Scale
for coast use
For the purpose of showing the forces of winds by wind roses on Meteorological Charts, winds are grouped as follows :—
Scale Numbers.
Calm.
..
..
..
..
0
Light winds.
..
..
..
1 to 3 ..
Moderate winds.
..
..
..
4 to 7 ..
Gales.
..
..
..
8 and above
* For statistical purposes force 7 is not reckoned as a gale.
t The fishing smack in this table may be taken as representing a trawler of average type and trim. For larger or smaller boats and for special circum"
stances, allowance must be made.
Calm
Light air
Beaufort's description of
wind, International
0
1
Beaufort
Number,
International
W
C/3
O
>
f
w
BLACK-BULB THERMOMETER
38
Black-bulb Thermometer.—A mercurial maximum thermometer with
blackened bulb, mounted in an evacuated outer glass sheath and exposed
horizontally to the sun's rays for the purpose of ascertaining the maximum
temperature " in the sun." On account of the difficulty of obtaining comparable
results with different instruments and of interpreting the indications of an
individual instrument, black bulb thermometers are not now recommended
as a means of measuring solar radiation. Such measurements are carried out
with a PYRHELIOMETER.
Blizzard.—A term originally applied to the intensely cold north-westerly
gales accompanied by fine drifting snow which may set in with the passage
of a depression across the United States in winter.
The term has come to be applied to any high wind accompanied by great
cold and drifting or falling snow, especially in the Antarctic, where, however, the
blizzards often cause a rise of temperature by removing the surface layer of
very cold air formed during calms.
Blood-rain.—Rain of a red colour which leaves a red stain on the ground.
The colouration is due to the drops containing small dust particles which
have been carried by currents in the upper air, sometimes for long distances,
from some sandy region. The phenomenon has been most frequently observed
in Italy, but it has been known to occur in this country.*
Blue of the Sky.—The blue of the sky is due mainly to the scattering of
sunlight by the individual molecules constituting the air. It was demonstrated
mathematically by the fifth Lord Rayleigh that if light of different wave­
lengths were to pass through a medium containing very small particles the
proportion to be scattered would be greater the shorter the wave-length of
the light. Thus the short waves composing the blue and violet end of the
spectrum are scattered more than the long red and yellow waves. Hence
light passing through a medium containing a great number of such particles
is left with an excess of red, whilst light emerging laterally has an excess of
blue. It is for this reason that soapy water looks yellowish when one looks
through it at a source of white light and bluish when one looks across the
direction of illumination.
That a column of air viewed across the direction of illumination also
appears blue was demonstrated in the laboratory by the present Lord Rayleigh.
Accordingly there is no need to assume the existence of grosser particles
suspended in the air to explain the normal blue of the sky. Where large
particles due to smoke or dust are present the light scattered by them does not
contain so high a proportion of blue, in other words the blue of the sky is
diluted with white. The sky as seen from high mountains is of a deeper but
purer blue because there are fewer large particles in suspension than at lower
altitudes.
For systematic comparisons of the depth of colour in the blue sky, a scale
has been devised by F. Linke, of the Meteorologische-Geophysikalische
Institut, Frankfurt. On this scale white is 0 and ultramarine blue is 14.
The scale is used in book form. By this means sky colour can be compared
with other variables such as the amount of dust in the atmosphere.
Bolometer.—An instrument for the determination of the intensity of
radiation of a definite wave-length, employing a blackened conductor whose
change of resistance with temperature furnishes a measure of the quantity
required. The instrument is largely employed in the investigation of the
distribution of energy in the spectrum, especially in the infra-red region.
Board of Trade Unit.—See KILOWATT-HOUR.
* See London, Quart J.R. met. Soc., 28, 1902, pp. 229-52 and 30, 1904, pp. 57-91.
39
BRITISH SUMMER TIME (B.S.T).
Bora.—A cold, often very dry, north-easterly wind which blows, sometimes
in violent gusts and squalls, down from the mountains on the eastern shore
of the Adriatic. It is strongest and most frequent in winter, and on the
northern part of the shore. It occurs when pressure is high over central
Europe and the Balkans and low to the south over the Mediterranean. With
the establishment of an anticyclone to the north and north-east the bora
may continue for several days ; in such circumstances it shows a marked
diurnal variation, being at a maximum in the morning and a minimum about
midnight. During the bora the wind speed at Trieste has averaged over
80 miles per hour with gusts exceeding 125 miles per hour ; temperature has
fallen to 14° F., and humidity to 15 per cent. The bora is sometimes associated
with a depression over the Adriatic and is then accompanied by heavy cloud
and rain or snow.
Bouguer's Halo.—In the eighteenth century descriptions were published
independently by Bouguer and by Antonio de Ulloa of a phenomenon which
they had seen together on near-by clouds on the Andes. Each observer
reports seeing on many occasions his shadow (which we should call the
Brocken Spectre), rainbow-coloured rings round the shadow of his head
(i.e., a glory), and outside the coloured rings and somewhat removed from
them a fourth ring of pure white. This outer ring, which has been called
Ulloa's Ring, and also Bouguer's halo, was presumably a fog bow or white
rainbow. The theory of Bidhu Bhusan Ray* explains how the reflection
of light from small drops should produce simultaneously a glory and a white
rainbow. In Bouguer's description it is stated that the phenomenon was to
be seen only on clouds and even (meme) on those of which the particles were
frozen, not on drops of rain. It was, therefore, assumed that the outer ring
was produced by reflection from ice crystals. This idea is embodied in the
name Bouguer's halo, and numerous attempts have been made to determine
the shape of crystals which would serve to reflect the light in the right way.
It is likely, however, that even though crystals were present the clouds which
Bouguer observed consisted mostly of supercooled water drops. There is
no mention of simultaneous observations of an ordinary halo round the sun,
and Bouguer's wording implies that on some occasions at least the particles
were not frozen.
Boys Camera.—A special type of camera first introduced by Sir Charles
Boys for the study of LIGHTNING. Originally, two lenses were rapidly rotated
at opposite ends of a diameter of a circle and gave two distorted photographs
of a flash of lightning. From these the direction of the discharge and its
velocity of travel could be calculated. A later development was to have the
lenses fixed and the film mounted on a large and rapidly rotating drum.
Brave West Winds.—A nautical expression denoting the prevailing westerly
winds of temperate latitudes. The region of westerly winds of the southern
hemisphere is termed the ROARING FORTIES.
Breeze.—A wind of moderate strength. (See BEAUFORT SCALE.)
The word is generally applied to winds due to CONVECTION, which occur
regularly during the day or night; of this type the following may be
mentioned—LAND AND SEA BREEZES, MOUNTAIN BREEZE, VALLEY BREEZE,
GLACIER BREEZE.
British Summer Time (B.S.T.).—The standard of time in common use and
that to which all legal and business transactions are referred in the British
Isles during a period in summer which is denned by the Summer Time Act,
1925. British summer time is one hour in advance of Greenwich mean civil
time, so that 9 h. G.M.T. is the same as 10 h. B.S.T.
* Calcutta, Proc. Indian Ass. Cult. Set. .8, 1923, pp. 23-46.
BROCKEN SPECTRE
40
Summer time was first introduced in 1916. From 1916 to 1925 the periods
during which summer time was in operation were fixed year by year by Orders
in Council and were as follows:—
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
From— May 21 Apr.8 Mar.24 Mar.30 Mar.28 Apr.3 Mar.26 Apr 22 Apr. 13 Apr. 19
to
— Oct.l Sept.17 Sept.30 Sept.29 Oct.25 Oct.3 Oct.8 Sept.16 Sept.21 Oct.4
From 1926 to 1938 the period of summer time in Great Britain, Northern
Ireland, the Channel Isles and the Isle of Man began at 2 h. Greenwich mean
time on the morning of the day next following the third Saturday in April
(or if that day was Easter Day the day next following the second Saturday
in April) and ended at 2 h. Greenwich mean time (3 h. B.S.T.) on the morning
of the day next following the first Saturday in October.
From 1939 the periods during which summer time was in operation were
again fixed year by year, and summer time was kept continuously in operation
from February 25, 1940 to October 7, 1945. Double summer time was first
introduced in 1941 and the periods when the clocks were kept two hours
ahead of Greenwich mean time were fixed year by year.
The times when summer time and double summer time (marked with an
asterisk) began and ended are as follows :—
1939 1940 1941
From—
Apr. 16 Feb.25 May4*
1942
1943
1944
Apr.5* Apr.4* Apr.2*
to—
Nov.19 Dec.31 Aug.10* Aug.9* Aug.15* Sept.17*
1945
1946
1947
1948
1949
Apr.2* Apr. 14
Mar. 16 Mar. 14 Apr.3
Apr. 13*
JulylS* Oct.6
Oct.7
Aug.10* Oct.31 Oct.30
Nov.2
In order that the statistics for the diurnal variation of meteorological
phenomena may not be affected by the introduction of the disturbing effect
of summer time, and to facilitate the international exchange of information,
meteorological observations are normally made at fixed hours by G.M.T.
throughout the year.
Brocken Spectre.—When an observer stands on a hill partially enveloped
in mist and in such a position that his shadow is thrown on to the mist he
may get the illusion that the shadow is a person seen dimly through the mist.
The illusion that this person or " spectre " is at a considerable distance is
accompanied by the illusion that he is gigantic. The Brocken is a hill in
Germany. See GLORY.
Brontometer, from bronte, a thunderstorm, a combination of apparatus
for following and noting all the details of the phenomena of weather during
a thunderstorm.
Bruckner Cycle.—A recurrence of periods of cold and damp alternating
with warm and dry years, the average interval between two successive
maxima being 35 years, though individual cycles may range in length from
25 to 50 years. A belief in a cycle of 35 to 40 years in Holland was known to
Sir Francis Bacon in 1625 but the periodicity was rediscovered in 1890 by
A. Bruckner, who calculated its average length as 34-8 years. Since then
periodicities of about this length have been found in many series of meteoro­
logical data. The last epoch of cold and rain was due in Europe about 1916
to 1920, but the amplitude is not large enough to dominate the irregular year
to year variations.
Brush Discharge (Electricity).—See ST. ELMO'S FIRE.
Bumpiness.—A word used to define the sensation experienced when flying
in an unstable atmosphere. A state of turbulence of the atmosphere, in which
both ascending and descending currents occur. Bumpiness may be due to
vertical currents, set up as the result of irregular heating of the earth's surface,
or to air streams following the well-defined contours and irregularities of the
ground. It may extend to considerable altitudes, but generally it is greatest
41
CALENDAR
in the lower layers up to about 3,000 ft. It varies with the character and
strength of the wind, and is more common over the land than over the sea.
Surface irregularities seldom affect the upper air to a height greater than three
to four times that of the obstruction. See AIR POCKETS.
Buoyancy.—A term used to denote the load, including the envelope and
fittings, which can just be supported by a balloon or an airship. This buoyancy
arises from the difference between the density of the light gas inside the
envelope and the heavier air outside. The vessel will just rest in equilibrium
when the total weight is the same as that of the air displaced ; the load which
can just be supported is thus the difference between the weight of the gas in
the envelope and that of the volume of air displaced.
Buran.—A strong
It is most frequent in
snow, but strong NE.
snow-bearing wind is
NE. wind which occurs in Russia and central Asia.
winter, when it is very cold and often raises a drift of
winds in summer are also termed buran. The winter
also termed poorga.
Buys Ballot's Law.—A study of synoptic charts caused Professor Buys
Ballot, of Utrecht, to enunciate, in 1857, the law which states that if, in the
northern hemisphere, you stand with your back to the wind, pressure is lower
on your left hand than on your right, whilst in the southern hemisphere the
reverse is true. Meteorologists often put the same law in a slightly different
way by saying that in the northern hemisphere winds go anticlockwise around
low-pressure centres and clockwise around high-pressure centres, the reverse
holding true in the southern hemisphere.
Calendar.—The word is derived from the Roman " Kalends," and
connotes an arrangement of time into ordered sequences of hours, days,
months, years, etc. Many attempts to secure simple calendars have been
made and the form of the. calendar has been frequently changed. There are
three principal natural periods of time which have been used for the purposes
of the calendar from time immemorial, viz., the (solar) day, the month and
the year ; corresponding respectively and more or less exactly with the mean
period of rotation of the earth in reference to the sun, the period of the
revolution of the moon round the earth (about 29£ days), and the period
required by the earth to revolve in its orbit around the sun (365 days, 5 hours,
48 min. and 46 sec.). Difficulties in constructing calendars arise from the
facts that the year is an exact multiple neither of the solar day nor of the period
of the moon's revolution.
The early history of the calendar, and of the origin of the names of the
days of the week and of the months is interesting, but need not be pursued
here. The most recent great reform of the civil calendar was that carried out
by Pope Gregory XIII in 1582. The Gregorian calendar now used for civil
purposes by all nations, superseded that introduced by Julius Caesar, who was
the first to give effect to the general principal of regulating the civil year by
the sun rather than the moon. At the time of Caesar an error of three months
had arisen in the dating of the vernal equinox, then fixed at March 25. He
corrected this error by the intercalation of 90 days into the current year
(the last " year of confusion ") and fixed the date of commencement of the
first Julian year as January 1 of the 708th year after the founding of Rome
(equivalent to 46 B.C.). He also ordained that the " odd " months, January,
March, May, July, September and November, should each consist of 31 days,
and the other months 30 days, except February, which in common years should
have 29 days and every fourth year 30 days. This simple and sensible arrange­
ment was subsequently altered to gratify the vanity of Augustus, by giving
the month bearing his name 31 days. To effect this, a day was taken from
February and added to August, and then to avoid the inconvenience of
having three successive months with 31 days the durations of the last four
months were altered to their present values.
(90565)
B*
CALIBRATION
42
The Julian calendar was based on a solar year of 365J days, which exceeds
the true value by 11 minutes 14 seconds. This involved an error of one day
in 128 years. The reform introduced by Pope Gregory XIII in 1582 aimed at
fixing the date of the vernal equinox as March 21. He corrected the
accumulated error of the Julian calendar by suppressing ten days, and also
ordained that in future centurial years should be ordinary years unless they
were divisible by 400 without remainder. Thus 1600 would be a leap year,
but 1700, 1800 and 1900 would be ordinary years. This refinement reduces
the error to one day in 3,323 years. The Gregorian calendar (known as New
Style) was introduced into Rome, Spain, Portugal and part of Italy in March
1582, and had become generally current in Europe by 1700. In Great Britain
the change was made in 1752. It was ordained that the day following
September 2 in that year should be called the 14th of that month, and that the
Gregorian system of fixing leap years should in future be adopted.
For meteorological purposes it is usual to adhere to the civil calendar, and
to publish summaries of climatological data for ordinary civil months or weeks.
For certain purposes, however, there are advantages in selecting an epoch
other than January 1 as the date of commencement of the year. Thus the
" farmer's year " adopted for the Weekly Weather Report begins on the Sunday
nearest to March 1, while for the purpose of the Crop Weather Scheme, a
" grower's year " beginning November 6 has been adopted at the suggestion
of Sir Napier Shaw. The " water year " beginning October 1 has been adopted
for the summarization of data of surface and underground water by the Inland
Water Survey (Ministry of Health).
Calibration.—Originally the name given to the process of finding the
calibre (or area of cross-section) of a tube. When a tube is not of uniform
cross-section, marks so spaced that the volume of the tube between consecutive
marks is everywhere the same, are calibration marks. The use of the word
has been greatly extended, e.g. the calibration of the Dines meteorograph
consists of making marks on the recording plate corresponding with various
pressures, the temperature being kept constant, and repeating the process
at other temperatures so as to obtain a framework of isothermal and isobaric
lines that indicate the pressure and temperature corresponding with any point
on the record.
Calm.—Absence of appreciable wind. Smoke rises vertically. On the
BEAUFORT SCALE OF WIND FORCE calm is accorded the figure 0.
Calorie—or gram-calorie.—The heat required to raise the temperature
of 1 gram of water by 1° A. at 288° A. The calorie is often used in connexion
with measurements of solar radiation. Thus the SOLAR CONSTANT is usually
expressed as having a mean value of 1-93 calories per square centimetre per
minute.
A " large calorie " or " kilogram-calorie " is the heat required to raise
the temperature of 1 kilogram of water by 1° A. at 288° A.
1 gram calorie=4 • 18 joules =4• 18X 10T ergs=4 • 18 watt-sec. = -004 British
thermal units.
Cap.—A name frequently given to the transient patches of cloud which
sometimes form on or just above the tops of growing cumulus clouds, and are
soon absorbed into them. It is also used for clouds on hill tops. It is technically
known as pileus. See LENTICULAR.
Catchment Area.—Defined for administrative purposes as the area within
the jurisdiction of a Catchment Board under the Land Draining Act of 1930.
The term is also commonly used with the same meaning as DRAINAGE AREA.
Celsius, Anders.—A Swedish astronomer and physicist, born at Upsala
November 27, 1701. He held the chair of astronomy at the University of
his native town from 1730 until his death in 1744. His thermometer was
first described in a paper to the Swedish Academy of Science in 1742. He
divided the interval between the freezing and boiling points of water into
43 CINEMATOGRAPHY, METEOROLOGICAL
100 parts, the lower fixed point being marked 100. The present system whereby
the freezing point is marked 0 and the boiling point 100 was introduced by
Christin, of Lyons, in 1743.
This particular scale is now generally referred to as the CENTIGRADE
scale or the Celsius scale, and sometimes the centesimal scale.
Centigrade.—A modification of the thermometric scale introduced by
ANDERS CELSIUS. It has its zero at the melting point of ice, while 100°
represents the boiling point of pure water at a pressure of 760 mm. of mercury.
A centigrade degree is f of a Fahrenheit degree. The Centigrade scale and the
Absolute scale are alike to the extent that on both the interval between the
freezing and boiling points of water is divided into a hundred degrees.
See ABSOLUTE TEMPERATURE.
Centre of Action.—In discussing the abnormally cold winter of 1879-80
in France, Teisserenc de Bort concluded that the character of any winter
was determined by the relative positions and intensities of the Icelandic
" low " and the Azores and Siberian anticyclones. He accordingly (in 1881)
designated these and similar areas of low and high pressure as " the great
centres of action of the atmosphere." The term was brought into general
use by the writings of H. H. Hildebrandsson ; originally it was applied to
depressions and anticyclones seen on an ordinary daily synoptic chart, as
well as to those more permanent lows and highs represented on charts of monthly
averages, but the former, transitory features are not now generally included
in the term " centre of action." On the other hand, the term is sometimes
extended to include areas such as South America or the Mediterranean, which
are not occupied by semi-permanent lows or anticyclones, but where the
barometric pressure is closely correlated with the contemporary or subsequent
pressure of many other parts of the world. The idea may be extended still
further to include areas where the important element is not pressure, but
temperature, rainfall or ice. Sir Gilbert Walker defines centres as " active "
if they are highly correlated with subsequent conditions at other centres, and
" passive " if they are correlated with preceding conditions.
Chart.—A map of an area of the earth showing lines of latitude and
longitude, and the more important physical features of the area (sometimes
only the coast line appears), upon which is superposed a representation of the
distribution of some physical quantity over the area. Examples are weather
charts, isobaric charts, charts of ocean currents, etc. Sometimes the word
chart is applied also to what are more properly termed diagrams or graphs.
Chinook.—A warm dry wind, similar in character to the FOHN, which
occurs on the eastern side of the Rocky Mountains. It blows from the west
across the mountains and is warmed adiabatically ; it usually occurs suddenly,
a large rise in temperature takes place, and the snow melts very rapidly.
Cinematography, Meteorological.—In recent years the motion picture
camera has been added to the list of apparatus available to the meteorologist
for the purpose of giving a continuous record of observations. The most
striking results have been achieved in recording the changing structure of
cloud in the process of development or dissipation. By the choice of a suitable
ratio of camera speed to ultimate projection speed the movements of the
cloud can be shown speeded up to any desifed extent. An example of the
result produced is provided by a picture of cumulus cloud in continuous
everchanging movement as though it were a huge mass of steam. Great
clarity and revelation of detail are obtained by the use of modern photographic
technique. Professor Mugge has made a series of meteorological films in
Germany illustrating the development of clouds in this manner, and there
are a few such films in England incorporating sections employing a similar
technique.
Apart from its use as an instrument for recording the changing structure
of actual cloud the cin6-camera has been employed in the research laboratory
(90565)
B*2
CIRCULATION OF THE ATMOSPHERE
44
to record observations more fully than could otherwise have been done. In
particular it has been employed to record the changing patterns formed by
artificially produced cloud to support views put forward in explanation of
particular types and patterns of cloud found in the atmosphere. A special
advantage of the film record of any series of events such as takes place in the
movements of a cloud is that there is no limit to the number of times which the
same movements can be studied since the film can be projected as often as
necessary.
Since the processes involved in meteorological phenomena are frequently
complicated the film is a particularly suitable medium of instruction because
the ideas it is desired to convey must first be simplified so as to be readily
assimilable in the form of pictures. A number of educational films on
meteorological subjects have been made in England.
Circulation of the Atmosphere.—See ATMOSPHERIC CIRCULATION.
Cirrocumulus (Cc.).— A cirriform layer or patch composed of small white
flakes or of very globular masses, without shadows, which are arranged in groups
or lines, or more often in ripples resembling those of the sand on the sea shore.
In general cirrocumulus represents a degraded state of cirrus and cirrostratus
both of which may change into it. In this case the changing patches often retain
some fibrous structure in places.
Real cirrocumulus is uncommon. It must not be confused with small alto­
cumulus on the edges of altocumulus sheets.
Cirrostratus (Cs.).— A thin whitish veil, which does not blur the outlines of
the sun or moon, but gives rise to halos. Sometimes it is quite diffuse and merely
gives the sky a milky look ; sometimes it more or less distinctly shows a fibrous
structure with disordered filaments.
Cirrus (Ci.).—Detached clouds of delicate and fibrous appearance, without
shading, generally white in colour, often of a silky appearance.
Cirrus appears in the most varied forms such as isolated tufts, lines drawn
across a blue sky, branching feather-like plumes, curved lines ending in tufts,
etc. ; they are often arranged in bands which cross the sky like meridian lines,
and which, owing to the effect of perspective, converge to a point on the horizon,
or to two opposite points (cirrostratus and cirrocumulus often take part in the
formation of these bands).
Clear Sky, Day of.—In a resolution adopted at the International
Meteorological Meeting at Utrecht in 1874, a " day of clear sky " was defined
as one on which the average CLOUDINESS at the hours of observation is less
than two-tenths of the sky.
On January 1, 1949 OKTAS were first adopted, and since then a " day of
clear sky " is defined in Great Britain as one on which the average CLOUDINESS
at the hours of observation is less than 2 oktas of the sky.
Climate.—The average weather conditions of any locality. The climate
of a locality is governed by three factors, its latitude (see CLIMATIC ZONES),
its position relative to the continents and oceans, and the local geographical
conditions. The interiors and eastern parts of the great continents generally
have a small rainfall, a low humidity and a great range of temperature, both
from day to night and from summer to winter, giving a continental climate.
Oceanic islands and the western parts of the continents generally have a
heavier rainfall, a greater humidity, and a more uniform temperature,
forming an oceanic or insular climate, but there are numerous exceptions,
as on the western coasts of continents in sub-tropical latitudes, which have
a desert climate. Local climates are modified by altitude above sea level and
by the neighbourhood of mountain ranges or large lakes. Great towns are
usually characterised by a local deficiency of bright sunshine due to the
presence of smoke. A climatic table should include data for each month and
for the year, for all the elements of weather which affect human health or
activity. Under temperature should be given the mean daily temperature,
the mean daily maximum and minimum, the average extreme values and the
45
CLIMATIC ZONES
highest maximum and lowest minimum temperatures recorded in each
month; temperatures at different depths in the soil are also useful if
available. Other data are the average relative humidity and amount of
cloud at different hours, the duration of bright sunshine, the average rainfall
with the maximum amount recorded in 24 hours, the frequencies of days of
rain, snow, hail, thunder, frost, fog and gale, and the frequencies of winds
from different directions, with the average velocity of the wind. Barometric
pressure is not an important climatic element except at great altitudes. The
table should be accompanied by a description of the characteristic weather of
the different seasons, the average and extreme dates of first and last frosts and
falls of snow, the duration of the snow cover, or in regions with definite wet
and dry seasons, the onset of the rains. In some places the weather goes
through a fairly regular series of daily changes, especially in connexion with
the alternation of land and sea breezes, and this should be noted.
Climatic Changes.—During geological ages, the climate of any area such
as England has not remained constant, but has varied over a wide range.
Twenty million years or so ago the climate of England was warmer than at
present, but a few hundred thousand years ago the climate was glacial. It
is now generally admitted that there have been variations of temperature and
rainfall in north-western Europe since the close of the ICE AGE, and that
about 5,000 to 3,000 B.C. the temperature was higher than at present,
forming the " Climatic Optimum," while a period of cold rainy weather
began about 850 B.C. There are indications of a dry period in the north
temperate belt in the sixth to eighth centuries, and of a period of great storminess
and heavy rainfall in the twelfth to fourteenth centuries, but the changes
during the Christian era are not universally accepted. The principal evidence
for them is, in Europe, fluctuations of Alpine glaciers and in the traffic across
Alpine passes ; in Asia, variations in the level of the Caspian Sea and other
salt lakes ; in North America, variations in the rate of growth of the Sequoias
of California, some of which are more than 3,000 years old. As a rule
instrumental meteorological observations do not reveal any indications of
climatic changes, but the series of rainfall measurements in England, which
have been standardised over a period of 200 years, point to a pronounced dry
period in the first half of the eighteenth century. A number of theories have
been put forward as to the causes of geological changes of climate, including
changes of solar radiation, variations in the elements of the earth's orbit,
changes in the land and sea distribution, mountain building and volcanic
eruptions.
Climatic Zones.—The word CLIMATE is derived from a Greek word meaning
" to incline," and the original zones of climate were zones in which the
inclination of the sun's rays at noon was the same ; that is, zones of latitude.
The accumulation of meteorological data has shown that winds and rainfall,
as well as temperature, have a zonal arrangement, but that the true climatic
zones do not run strictly parallel with lines of latitude. Eight principal
zones are distinguished :—near the equator a zone of tropical rain climate,
then two sub-tropical zones of STEPPE and DESERT climate, then two zones
of temperate rain climate and, in the northern hemisphere only, an incomplete
zone of boreal climate with a great annual range of temperature ; finally,
two polar caps of snow climate. The equatorial zone is divided into the
equatorial rain-forest zone, which extends over the Atlantic and Pacific Oceans
as the DOLDRUMS, with rain in all seasons, and a belt of Savanna climate on
either side, with a well-marked alternation of dry and rainy seasons, the latter
occurring in the " summer " months. The sub-tropical zones include most
of the world's great deserts—the Sahara and Arabia, Arizona, the Kalahari and
the deserts of South America and Australia ; over the oceans they include
the TRADE WIND belts and the HORSE LATITUDES. The temperate zones are
divided into the Mediterranean climates with mild rainy winters and hot
dry summers, and the temperate rain belts with rain in all seasons. On
the eastern margins of the continents, especially in Asia, the sub-tropical
CLIMATOLOGY
46
desert zone and the Mediterranean climate are replaced by areas with a
MONSOON climate. For a detailed classification of climates consult " Die
Klimate der Erde," by W. Koppen, Berlin and Leipzig, 1923.
Climatology.—The study of CLIMATE.
Climatotherapy.—The treatment of disease by suitable climatic environ­
ment. Such environment is often to be found in so-called health resorts but
not exclusively, as even the warmth and shelter of large towns, for instance,
may be advantageous in certain cases. Owing to the seasonal variations of
climate the required conditions may have to be sought in different localities
at different times of the year. In brief, Climatotherapy may be described as
applied bioclimatology.
A considerable amount of literature on medical climatology has appeared
in recent years, one of the most comprehensive works being the " Trait6 de
climatologie biologique et medicale, "* a publication of over 2,600 pages in
three volumes to which some 140 collaborators contributed.
It may be of interest to recall that the second part of the word is con­
nected with the Greek for " service " or " nurture," so that etymologically
Climatotherapy is capable of a far wider interpretation than that of the usual
definition.
Cloud-burst.—A term commonly used for very heavy rain, usually
associated with thunderstorms. Extremely heavy downpours are sometimes
recorded, which in the course of a very short time tear up the ground and fill
up gulleys and watercourses ; this may happen at any place, but it occurs
frequently in hilly and mountainous districts, where it may sometimes be due
to the sudden cessation of convectional movement, caused by the supply of
warm air from the lower part of the atmosphere being cut off as the storm
moves over a mountain range. With the cessation of the upward current,
the raindrops and hailstones which it had been supporting must fall in a much
shorter time than they would have done had the ascensional movement
continued.
Cloudiness.—Amount of sky covered by cloud, irrespective of the type
of cloud. It is estimated by eye-observation and usually expressed by a
scale of eighths (OKTAS) of sky covered. On this scale 8 represents a sky
totally covered by cloud in which no particles of blue sky are visible, and 0 an
entirely cloudless sky. Observers are usually advised to subdivide the sky
mentally into four quadrants, and to combine estimates of the amount of
sky covered in each quadrant.
Charts showing the distribution of mean cloudiness over the earth in
each month and the year are given in the " Manual of Meteorology," Vol. 2,
by Sir Napier Shaw (Cambridge University Press, 1928).
Clouds.—The number of forms which clouds may take is almost infinite,
but for purposes of description it is necessary to adopt some kind of
classification, though whatever classification is used there must at times be
border-line cases when a cloud seems to fall half-way between two classes and
perhaps to belong to neither.
The systems of classification which have been proposed have sometimes
been based on the observed appearance of the cloud and at other times on
the supposed method of formation. There can be no doubt that the former
is the correct method since an observer is able to judge definitely of the
appearance while the method of formation of a given cloud must be to some
extent a matter of opinion. The international cloud classification is based
upon the appearance of clouds and consists of 10 forms which for convenience
are sometimes separated into three classes of high clouds, medium clouds
and low clouds as follows :—
High clouds—cirrus, cirrostratus, cirrocumulus.
Medium clouds—altocumulus, altostratus.
Low clouds—stratocumulus, nimbostratus, cumulus, cumulonimbus,
_____stratus.______________________________________
* By Piery et alii.
Paris (Masson et Cie), 1934.
To face page 46.
FIG. 8.—Cirrus clouds, thread or feather clouds at a height of from
five to six miles, and generally of a white colour. They are composed
of ice crystals.
The picture gives an idea of rather more massive structure than
is usual with cirrus clouds, but the sweeps and wisps are very
characteristic.
(90565)
FIG. 9.—Altostratus.
FIG. 10.—Altocumulus.
FIG. 11.—Stratocumulus from an aeroplane.
FIG. 12.—Stratocumulus from below.
FIG. 13.—Cumulonimbus (Thunder-cloud) with "Anvil "
extension of cirrus nothus, October 30, 1915.
FIG. 14.—Mammillated clouds (adjective mammatus).
FIG. 15.—Cumulus protruding through stratocumulus.
47
CLOUDS
The definitions of these various cloud forms will be found under the indi­
vidual names. Some general remarks on clouds and their method of formation
are given here. The heights of clouds, even of one type, vary within very
wide limits, and " high clouds " of the above classification may sometimes
be found below " medium clouds." The level at which high clouds are
generally found is between 25,000ft. and 35,000 ft., while medium clouds
are found between 10,000 ft. and 25,000 ft., and low clouds are usually below
10,000 ft. although the tops of large cumulus and cumulonimbus may rise
much above this level. These average heights refer to temperate latitudes.
The heights of clouds tend to be greater in summer than in winter, and
greater in the tropics than in high latitudes. Clouds are almost entirely
confined to the TROPOSPHERE and seldom penetrate into the STRATOSPHERE,
the base of which is on the average at a height of about 35,000 ft. over the
British Isles, though it is as high as 45,000 ft. in extreme cases.
Apart from surface fogs, clouds are due mainly to the air being cooled
below its DEW-POINT by a reduction of pressure (see ADIABATIC), and by far
the most effective cause of this is upward movement. There are three main
types of vertical movement, and observations of the three-dimensional struc­
ture of clouds, which aviation has rendered possible, show that many of the
more important cloud forms can be grouped together according to the nature
of the vertical motion, in the following manner :—
(a) Slow upward movement of a large mass of air, due to orographic
causes or to CONVERGENCE in the horizontal motion of the air under­
neath. This process is responsible for clouds of altostratus and nimbostratus type and for continuous precipitation.
(6) Small air masses rising up through the atmosphere causing cumulus
or cumulonimbus clouds and showers.
(c) Churning up of a layer of air by small eddies, which increases the
moisture content of the upper part of the layer and may cause cloud
to be formed. This gives rise to horizontal cloud sheets, i.e., stratus,
stratocumulus, altocumulus, cirrocumulus.
In some cases two or three of these types of vertical motion operate
together, and there are no sharply defined limits which separate the one
from the other. If we try to trace out the life history of any particular
cloud with the factors which have operated in the more remote as well as
the immediate past to lead to its formation, we shall frequently find that
more than one of the above processes must be involved.
The clouds associated with the three methods of formation may be con­
sidered in greater detail:—
(a) Altostratus clouds are some thousands of feet thick, usually
continuous but occasionally in a succession of layers. If the clouds are
nebulous or fibrous in appearance, they normally consist entirely of ice
crystals or snowflakes. There is some reason to think that the blurred
appearance of the sun or moon (see Fig. 9) can only be produced by
ice-crystal clouds ; when seen through clouds of waterdrops, the edges
of the disc are sharp. There is often an ill-defined base to the cloud where
falling snowflakes evaporate or melt into rain. The clouds have no great
opacity, and the sun can usually be seen dimly through several thousand
feet of cloud.
Cirrostratus cloud is of the same type as altostratus but is higher
and more nebulous. It is frequently the advance guard of a large
cloud mass which is formed in a current of air rising up a surface of
DISCONTINUITY.
(6) Heap clouds : cumulus and cumulonimbus. Cumulus clouds are
normally formed when the LAPSE rate of temperature below them is
equal to the dry ADIABATIC rate (near the earth's surface this rate may
be exceeded) so that air slightly warmer than its surroundings rises up
like a bubble. They develop over the land on the majority of summer
They also form in a cold current
days, and dissolve in the evening.
CLOUDS
48
passing over a warmer sea surface. Once condensation takes place in a
mass of rising air, latent heat is set free and the air cools at the adiabatic
rate for saturated air. The growth of the cloud depends on whether the
lapse-rate of temperature in the surrounding air is greater or less than
the saturated adiabatic rate. If it is less, conditions are stable, since
the rising air becomes colder and therefore heavier than its surroundings
and tends to fall back again. The clouds then tend to assume flat shapes
and often spread out into a layer of stratocumulus or altocumulus. If,
however, the lapse-rate is greater than the saturated adiabatic the rising
air remains warmer and lighter than its surroundings so that conditions
are unstable, the clouds tend to tower upwards and develop into cumulo­
nimbus. The cumulonimbus clouds usually develop from large masses
or long banks of cumulus, and a group of clouds often amalgamates into
one large mass, the whole process being spread over a few hours. It is
not easy to fix the limit between large cumulus and cumulonimbus.
Probably the occurrence of heavy precipitation furnishes the best criterion,
but this cannot always be seen from a distance, and the development
of precipitation does not depend only on the height attained by the
cloud summit, which may considerably exceed 10,000 ft. without showers.
If the top becomes cirriform (fibrous), the cloud is certainly cumulo­
nimbus, but in the case of some heavy showers there is no cirriform top.
The tops of severe summer thunderstorms frequently reach a height of
25,000 ft., and probably sometimes 35,000 ft. or even more. The ANVILS
consist mainly of snow and may remain after the storms die out, in the
form of dense masses of cirrus or altostratus.
When the atmosphere is stable in the lower layers, but damp and
unstable for saturated air above, cumulus clouds, and often eventually
cumulonimbus, may develop from a layer of stratified clouds (formed
by process (a) or (c) or both combined). The cloud known as altocumulus
castellatus consists of a layer of small turreted clouds grouped like
ordinary altocumulus and formed in this way. It is also possible for
large cumulus or cumulonimbus to develop at relatively high levels,
independent of convection from the earth's surface. The base of cumulo­
nimbus may be above 10,000 ft., but in the British Isles such cases are
rare and thunder and lightning are unlikely to develop. Clouds developed
in a similar manner with a base at about 6,000 ft. may give rise to severe
thunderstorms.
(c) Flat cloud sheets : stratus, stratocumulus, altocumulus, cirrocumulus. There is an essential similarity of structure between all these
clouds, though the higher sheets are usually thinner than the lower, and
have smaller cloudlets. The lower sheets often exceed 1,000 ft. in thick­
ness, but layers less than 50 ft. thick may occur over limited areas at
any height, often with quite small ripples. As a rule there is a definite
arrangement of ripples or rounded cloudlets, and from above the clouds
often resemble the sea (Fig. 10). Even low stratus is frequently rippled
on its upper surface. The lower surface usually appears uniform, but
even if structure existed it might be difficult to see, owing to bad lighting.
If there are definite ripples or waves, the maximum condensation occurs
at the crests of the waves, where the air is subject to the maximum
decrease of pressure.
It is found that the lapse-rate of temperature is high below these
cloud sheets but relatively low just above them, often with an INVERSION
(see also SURFACE OF SUBSIDENCE). Such a condition is accompanied
by much TURBULENCE (i.e., a thorough churning up of the air by eddies)
below the clouds and very little turbulence above them, and this is
verified by observations of Dumpiness by aeroplane pilots. It can readily
be shown that this state of affairs favours the development and
maintenance of a cloud sheet.
Sometimes a single sheet of stratocumulus cloud causes dull weather
for several successive days, especially in winter anticyclones. On summer
to face Page 49
1016
AUGUST
23, 1928, 7h.
COL
49
COLD SECTOR
days, and in suitable conditions over the sea, independently of time
of day or season, there are frequently cumulus clouds below the
stratocumulus, whose tops may protrude through the latter (Fig. 15)
breaking up the regularity of the sheet to some extent.
Cirrus clouds cannot at present be adequately discussed from the point
of view of physical causes, though certain large tufts of great vertical extent
have evidently a cumuliform structure, whether or not they are developed
from the tops of cumulonimbus. A well-marked thread-like structure indicates
ice crystals, and sometimes cirrus clouds are formed by snow falling from
altocumulus clouds, which consist of supercooled water drops. There are
certain very delicate forms of cirrus and cirrostratus (with no long threads)
which may cause brilliant CORONAE and IRIDESCENCE, and probably consist
of very small spherical globules, and not of ice crystals. Cirrocumulus clouds
are also normally of this type.
Since 1943 radar has been used for the study of cloud structure. The
pulses of the ultra-short \vaves used in radar are reflected by clouds and
precipitation to a degree proportional to ND6 where N is the number of
droplets in unit volume and D their diameter. Parts of cumulonimbus
clouds give a strong reflection. Stratiform clouds give little, if any, reflection.
Clouds, Weight of Water in.—Measurements on the Austrian Alps of the
quantity of water suspended in clouds have given 0-35 gm./m3 to 4-8 gm./m. 3
The water suspended as mist, fog or cloud may be taken as ranging from
0-1 to 5 gm./m. 3 (see A. Wegener, " Thermodynamik der Atmosphare,"
Leipzig, 1911, p. 262).
Col.—The saddle-backed region occurring between two anticyclones and
two depressions, arranged alternately. Frequently on a weather map only
two anticyclones appear, then the col is the region of relatively low pressure
between them and may be likened to a mountain pass between two peaks.
In a similar way the col may appear when only two depressions are shown
on the weather map.
The wind circulations associated with the neighbouring anticyclones or
depressions bring light airs from very different directions into close contact
with one another in the col. In summer the weather which results is
occasionally brilliantly fine, but thunderstorms are frequent and may occur
in any part of the col. In winter conditions are apt to be dull or foggy.
The anticyclones, between which a col lies, are generally stationary or
slow-moving, and the col occupies a region through which an approaching
depression may readily pass. Hence, although a col does not usually move
rapidly in itself, it does not as a rule form an abiding feature on the pressure
map. (Plate II.)
Cold Front.—The boundary line between advancing cold air and a mass
of warm air under which the cold air pushes like a wedge. The surface of
separation is called the frontal surface and this meets the earth in the cold
front. Its passage is normally accompanied by a rise of pressure, a fall of
temperature, a veer of wind, a heavy shower and sometimes a LINE-SQUALL,
perhaps with thunder. The front is not always sharply denned, and sometimes
it is not clearly shown by surface temperatures, but only by upper air
temperatures, weather, wind and barometric tendencies. Occasionally the
barometer continues to fall behind the front, though at a much reduced rate.
The normal direction of slope of the frontal surface is at about 1 in 50
back from the cold front, but the point of the wedge may become rounded
owing to friction holding back the advance of the cold air on the surface
so that the direction of slope is reversed below about 2,000 ft. The
rate of advance of a cold front is roughly that of the component of geostrophic
wind at right angles to it. See DEPRESSION, FRONT.
Cold Sector.—That part of a DEPRESSION occupied by cold air on the
earth's surface, usually about half to three-quarters of a recently formed
depression, and the whole of an old one.
COLD WAVE
50
Cold Wave.—The fall of temperature associated with the air behind the
cold front of a passing depression. The term is used in a technical sense by
the United States Weather Bureau, with the meaning of a fall of temperature
by a specified amount in 24 hours, to a minimum below a certain temperature.
The amount of fall and the limit of minimum are different for different times
of the year and for different parts of the country.
Compass.—The magnetic or mariner's compass consists in its simplest
form of a graduated circular card at the centre of which a magnetized steel
needle is suspended so that it may turn freely in a horizontal plane. The
card is divided into 32 equal parts of 11J° each, called points. The points
are N., N. by E., NNE., NE. by N., NE. and so on. They are often called
orientation points. The compass needle points to the magnetic north (see
TERRESTRIAL MAGNETISM), not to the geographical north. At the present
time the magnetic north lies 13° to the west of true north in London and
18° to 19° west in the west of Ireland. Along two irregular agonic lines, one
passing through North and South America and the other through parts of
Europe, Asia and Australia there is at present no variation. In Arctic and
Antarctic regions the variation is very large and in places the direction may
even be entirely reversed, the north-pointing end of the needle indicating
the south.
Magnetic compasses have now largely given way to the gyrostatic compass.
This compass is one of several devices which utilise the property of a spinning
flywheel, of maintaining its axis in space. The gyro is maintained electrically,
and has been used in aeroplanes and on board ship, having the advantage of
being independent of both magnetic and electrical disturbances.
Compensation of Instruments.—An instrument designed to measure
changes in a particular physical quantity (e.g. pressure) may be affected
also by some other influence (e.g. temperature). To eliminate or minimise
the influence of the disturbing element, a device may be introduced for the
purpose of rendering the instrument insensitive to changes in the latter, in
which case it is said to be compensated. Thus, chronometers and aneroid
barometers are ordinarily compensated for temperature.
Component.—A word used to indicate the steps, in their various directions,
which must be compounded or combined geometrically in order to produce
a given displacement.
In the
o
diagram, AC is the geometrical sum
or resultant of the two components
AB, BC wherever B may be. It is not
necessary that the components should
be at right-angles to each other. The
above law of the composition of
displacements is applicable to the
composition of velocities, accelerations and forces.
Condensation.—The process of formation of a liquid from its vapour.
In a closed space wherever there is a free surface of ice or water, EVAPORATION
or condensation takes place until the water vapour exerts-a definite " pressure
of saturation," depending only upon the temperature, and not upon the
pressure of the surrounding air. This pressure of saturation is very much
greater at high than at low temperatures, as is shown in the table on p. 17.
Aitken has shown that condensation occurs when saturated air is cooled,
provided suitable nuclei are present, but not otherwise. In the atmosphere
such nuclei are rarely if ever absent. Condensation therefore occurs when air
is cooled below the dew point.
In the passage from the liquid to the gaseous state great quantities of
heat are absorbed, 539 calories for every gram of water evaporated at the
boiling point, and even more if the water is initially cold. Conversely much
heat is yielded up when condensation occurs, and this is an important factor
in controlling the rate of cooling.
51
CONDENSATION TRAILS
The principal causes of cooling, such as may lead to condensation, are :—
(1) contact with solid bodies which are at a lower temperature,
(2) dynamical cooling due to rarefaction,
(3) mixing with air at a lower temperature.
The result of (1) is most often visible as dew or hoar frost, while (2) and
(3) yield cloud or fog, and if carried far enough (2) will yield rain, snow, etc.
The physics of cloud formation and precipitation are still (1949) not fully
understood particularly as regards the development of RAINDROPS from the
very small droplets or ice crystals which are first produced in condensation.
It is very doubtful if growth depends upon the fusion of individual particles.
In 1933 Bergeron put forward a theory, based on the fact that the
saturation vapour pressure over water is greater than over ice, according to
which all raindrops are melted large ice crystals. The difference between the
saturation vapour pressures produces this effect because in Bergeron's view
in a cloud of mixed water droplets and ice crystals the crystals will tend to
grow at the expense of the water droplets and ultimately fall and melt when
air at above freezing point is reached.
Much work has been done since 1939 on condensation processes at below
freezing point in " cloud chambers " though the application of all the results
to the free atmosphere is uncertain. A valuable summary is given by
Dobson.* The details are complex but, briefly, as the humidity is increased
by cooling below freezing point only water droplets are formed down to
about —10° C., and below that temperature ice particles begin to form in
increasing numbers until below —41° C. condensation is mainly ice. An
important fact is that the ice particles form on natural nuclei, apart from
existing ice crystals, at these low temperatures at saturation with respect to
water. A few substances, e.g. cadmium iodide treated in a special way, are
known which can produce ice condensation at saturation with respect to ice,
but they do not occur naturally in the air so far as is known. In the
atmosphere ice particles have, however, been found at higher temperatures
than in condensation chambers, even at —4° C.
Since 1947 the effects produced by dropping solid carbon dioxide, a
procedure called " artificial nucleation " or " cloud seeding ", which has
a temperature not higher than —65° C., into clouds have attracted much
attention. Rain has been observed to fall from clouds treated in this way.
Most results have been obtained with clouds of supercooled water drops but
similar results have been claimed for clouds at above freezing point. The
process, at below freezing point, appears to be due to the formation of large
numbers of tiny ice crystals by the solid CO2 pellets in the saturated air
which then act as suggested by Bergeron. The process suggested by Langmuir
for the effects at above freezing point is a complex chain reaction described in
his General Electric Company Report, New York, 1948. The scientific
interest of these experiments is great but their practical use in rain-making
is, at least, very doubtful. An essential requirement is the existence of
naturally formed clouds developed to the point at which " seeding " just gives
the final impulse. Seeding may have a use in clearing gaps in anticyclonic
stratocumulus cloud.
Condensation Trails.—Persistent condensation trails are long, initially thin
clouds produced by aircraft engine exhausts when the humidifying effect of the
water vapour in the exhaust overpowers the opposed heating effect. For a given
engine there is for a given pressure a critical air temperature above which
trails cannot form. This theoretical result has been confirmed in practice.
The critical temperature is such that over Great Britain trails rarely form
below 28,000 ft. in summer and about 20,000 ft. in winter. In tropical areas
the heights are greater but in very cold conditions, such as winter in central
Canada, trails can form at ground level.
* DOBSON, G. M. B. ; Ice in the atmosphere.
p. 117.
Quart. J.R. met. Soc., London. 75, 1949
CONDUCTION
52
There is another non-persistent type of trail which may be formed at
wing tips and propellers of aircraft in damp air. The theory of persistent
trail formation does not apply to this type.
Conduction.—The process by which HEAT is transferred by and through
matter, from places of high to places of low temperature, without transfer
of the matter itself, the process being one of " handing on "of the heat-energy
between adjacent portions of matter. It is the process by which heat passes
through solids ; in fluids, although it occurs, its effects are usually negligible
in comparison with those of CONVECTION.
Constant.—A quantity which does not vary within the limits of space
or time (or both) under consideration. See also SOLAR CONSTANT and
RADIATION (Stefan's Constant).
Constant Pressure Chart.—A chart on which are drawn the synoptic
contour lines of the height above sea level of a selected isobaric surface in the
free atmosphere. Such charts are advantageous for the calculation of wind
velocity from the GRADIENT WIND relation since
y I D = g x slope of isobaric surface
where y is pressure gradient, D, air density and g, the acceleration due to
gravity. The slope is measured from the distance between the standard
contour lines. It is not necessary to know the air density as it is in using
constant level charts for the same purpose. Other elements, e.g. temperature,
winds observed at the given pressure, may be entered on the same chart.
Contessa del Vento.—A type of eddy cloud frequently found to the lee
side of isolated mountains. It is often observed over Mount Etna with a
westerly wind. In its most characteristic form it consists of a rounded base
surmounted by a protuberance directed up-wind.
Continentality.—In meteorology, a measure of the extent to which the
climate of any place is influenced by its distance from the sea. The influence
of continentality is most clearly shown on the diurnal and annual ranges of
temperature which are large in continental climates. Since water has a high
specific heat and still more because the exchange of heat goes to a much
greater depth in water than in land, the temperature of a water surface can
vary only slowly compared with the temperature of a land surface. The
latter absorbs and radiates heat readily, and its temperature can therefore
vary through a considerable range even within 24 hours. Measures of
meteorological continentality have been derived by Brunt and others, based
on either the mean daily range of temperature or the mean annual range
in relation to the latitude.
Contour Lines.—In synoptic meteorology this term generally means
synoptic lines of constant height of isobaric surfaces. Isohypses is sometimes
used. See CONSTANT PRESSURE CHART.
Convection.—In convection heat is carried from one place to another by
the bodily transfer of the matter containing it. In general, if a part of a
fluid, whether liquid or gaseous, is warmed, its volume is increased, and the
weight per unit of volume is less than before. The warmed part therefore
rises and its place is taken by fresh fluid which is warmed in turn. Conversely,
if it is cooled it sinks. Consequently, if heat is supplied to the lower part
of a mass of fluid, the heat is disseminated throughout the whole mass by
convection, or if the upper part is cooled the temperature of the whole mass
is lowered by a similar process.
There are two apparent and important exceptions in meteorology. Fresh
water, when below the temperature of 39-l°F., 277°A., expands instead of
contracting on being further cooled. Hence a pond or lake is cooled bodily
down to 39-1° but no further, as winter chills the surface before it freezes.
Secondly, heat applied to the bottom of the atmosphere may stay there without
being disseminated upwards when the atmosphere is exceptionally stable in
the circumstances explained under ENTROPY.
53
CORIOLIS ACCELERATION
Convectional Rain.—Caused by the heating of the surface layers of the
atmosphere which expand and rise, giving place to denser cool air. The
warm air is frequently heavily charged with moisture taken up from the ground
or vegetation. The THUNDERSTORM rain of the summer afternoon is typically
convectional rain. As in the case of OROGRAPHIC RAIN, condensation occurs
when the process of expansion under diminishing pressure has reduced the
temperature of the rising mass beyond its dew point. The intensity of the
rain in severe thunderstorms is much greater than that in any other type of
rainfall. In the British Isles the intensity for a few minutes in a few cases
has exceeded the rate of 10 in. per hour. (See also INSTABILITY SHOWERS,
RAINFALL.)
Convergence.—Consider a portion of the earth's surface, for convenience
a square with sides 100 miles long running north to south and east to west,
though the argument is applicable to any area. If the wind is uniform over
the whole area as much air will flow into it on the sides which face the wind
as flows out of it on the other sides. If, however, the wind is not uniform a
case may arise when more air flows in than flows out. There is then said to
be convergence. For example, suppose the wind is on the whole westerly.
If along the south side of the square the wind is blowing a little from
S. of W. while along the north side it is blowing from N. of W., there will
be an inflow along both these sides. The amount of air which flows out on
the eastern side of the square will, however, be equal to that flowing in on
the western side, assuming the wind to be the same in both regions. Taking
account of all four sides more air will therefore flow into the square than
flows out of it, and there will be convergence over the area. The air cannot
go on accumulating, and the excess will have to flow out at the top, thus
leading to a rising air current. It is this which gives convergence its import­
ance. Rising air expands and is cooled (see ADIABATIC). If the cooling is
continued long enough and the air contains moisture, this moisture will
condense and cause cloud and rain. Except in arid climates, prolonged
convergence of the lower currents of the atmosphere must therefore eventually
give precipitation. Further, since these layers hold more moisture than those
above, convergence without precipitation steadily increases the total water
content contained in the whole column of air over a given area. It is important
to note that geostrophic winds (see GRADIENT WIND) can give no appreciable
convergence, for if the wind obeys the geostrophic law the amount of air
which flows out of any area will be almost exactly equal to the amount which
flows into it. There are two main causes of convergence. One is surface
friction, which causes a flow of air near the ground across the isobars from
high to low pressure, resulting in convergence in certain conditions, notably
in the centre of a depression or along a trough of low pressure. Secondly,
convergence takes place into a region of falling barometer, owing to lack of
perfect balance between the wind and pressure gradient. This effect is smaller
at the earth's surface than the frictional inflow into a depression, but as it
extends much higher it is probably often more important. See also ISALLOBARIC WIND.
Coriolis Acceleration.—Any body moving relatively to the rotating earth
experiences an acceleration relative to the earth called, after its discoverer,
the Coriolis acceleration.
The Coriolis acceleration is twice the vector product of the angular velocity
of the earth and the velocity of the body. In symbols it is the vector given by
2O A V
Where O is the vector angular velocity of the earth, V is the velocity of the
body and A is the symbol of vector multiplication.
The magnitude of the acceleration is 2QVsin^ where ip is the angle
between the earth's axis and the velocity of the body. Its direction is
perpendicular both to the earth's axis and to the velocity of the body relative
to the earth, and is such that a positive rotation about it carries the earth's
axis towards the direction of relative velocity.
CORONA
54
Taking a rectangular co-ordinate system fixed at a point on the earth's
surface in north latitude 0, with the x axis to the east, the y axis to the north
and z axis vertical, the components of the Coriolis acceleration are
2 Q, (w cos 0 — v sin <f>)
2 O u sin ^
— 2 O u cos <£
where w, v, w are the component velocities of the body. If there is no vertical
component of velocity the horizontal component of the acceleration is at
right angles to the velocity and directed to the left in the northern hemisphere,
and to the right in the southern hemisphere.
Corona.—A series of coloured rings surrounding the sun or moon. The
space immediately adjacent to the luminary is bluish white, while this region
is bounded on the outside by a brownish red ring, these two together forming
the aureole. In most cases the aureole alone appears, but a complete corona
has a set of coloured rings surrounding the aureole, violet inside followed by
blue, green, yellow to red on the outside. The series may be repeated more
than once, but the colours are usually represented merely by greenish and
pinkish tints.
The corona is produced by DIFFRACTION of the light by waterdrops. If
the colours are pure it is an indication that the drops are uniform in size.
The radius of the corona is inversely proportional to the size of the droplets.
Thus a corona, whose size is increasing, indicates that the water particles
are diminishing in size.
A corona is distinguished from a halo which is due to REFRACTION by
the fact that the colour sequence is opposite in the two, the red of the halo
being inside, that of the corona outside. In applying this criterion it must
be remembered, however, that the dull red which is the first notable colour in
the aureole ranks as outside the bluish tint near the luminary. An alternative
criterion is that the colours of the halo are at the inner edge of a luminous
area, whilst those of a corona are at the outer edge.
The term corona is also used to describe a particular form of AURORA
present only in the most active displays. When there are bundles of auroral
rays along the lines of force of the earth's magnetic field the effect of perspective
makes them appear to converge to the magnetic zenith.
Correction.—The alteration of the reading of an instrument in order to
allow for unavoidable errors in measurement. The measurement of nearly
all quantities is an indirect process, and generally takes the form of reading
the position of a pointer or index on a scale. When we wish to know the
pressure of the atmosphere we read an index on the scale of a barometer ;
when we wish to know the temperature of the air we read the position of the
end of a thread of mercury in a thermometer.
Almost all measurements are ultimately reduced to reading a position
or length on a graduated scale. Generally speaking, the reading depends
mainly on the quantity which the instrument is intended to measure, but
also partly upon other quantities. Thus the readings of barometers are
generally affected by temperature as well as pressure, those of thermometers
by alterations in the glass containing the mercury or spirit.
In most cases the amount of the error has to be determined and allowed
for by a suitable " correction." An ANEROID BAROMETER is often compen­
sated for temperature (see COMPENSATION OF INSTRUMENTS), and for a
mercury barometer the effect of temperature is made out and tabulated and
a correction introduced, which is derived from a table, when the temperature
of the " ATTACHED THERMOMETER " has been noted. In a similar manner
the correction of a barometer reading for the variation of GRAVITY at different
55
CORRELATION
parts of the earth's surface is worked by means of tables, the variation of
gravity with latitude having been previously reduced to a formula, by means
of observations from which the figure of the earth has been determined and
the change of gravity has been ascertained.
In some measurements, such as the determination of height by the use
of an aneroid barometer, corrections are numerous and complicated ; the
unconnected reading may even be only a rough approximation not sufficiently
accurate for practical purposes.
Correlation. —The method of correlation is a mathematical process for
determining the degree of relationship between two variable quantities or
varieties. For example, it is found that when pressure at sea level at any
place in Europe is unusually high, pressure at a height of 9 Km. above that
place is generally high also. Let xlt xz, . . . xn be the departures of
n values of pressure at sea level from their mean, and yv y z. . . • yn the
corresponding departures of the pressures at 9 Km. The average variation
of the y's about their mean is smaller than the average variation of the
x's, but we can make the figures comparable by dividing each series by its
" standard deviation " given by the expression : —
<**=
The square of the standard deviation, axz = 2 (x2) / n, is known as the
variance.
If now the variations of x were strictly proportional to the values of y,
knowing the one we could calculate the values of the other exactly, by
means of the relationship : —
..... (2)
x/aa =y/ov
but we can
meteorology,
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Such exact relationships rarely if
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the
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strictly proportional to the variations of y, and another part e. which is
independent of y. That is to say, we can write :—
..... (3)
x I ox = r y I av + e
The figure r is a measure of the closeness of the relation between the two
quantities x and y. It is accepted that the best value which can be given
to r is that which gives the smallest possible figure for the sum of the
squares of the values elt e2 . . . en resulting from the substitution of
xlt x2 . . . xn and ylt y2 . . . yn in (3), and this value of r is known as
the Correlation Coefficient. When the two quantities x and y are exactly
proportional, e is always zero, equation (3) reduces to (2) and r becomes
+ 1. On the other hand if the two quantities are inversely proportional,
e is again zero, but r is — 1. If there is no relationship at all between
x and y, r is zero and e = x. The correlation coefficient obtained by the
late W. H. Dines between pressure at sea level and that at 9 Km., was
+0-68.
The value of the correlation coefficient r is given by the expression : —
(4)
where 2 (xy) is the sum of the products xlt ylt xz yz . . . xn yn ; n is the
number of observations and ax, ay are the standard deviations of x and y
obtained by (1).
The relation between x and y is generally expressed in the form : —
..... (5)
x = bw y
where bxy = r a* / ay
Equation (5) is the REGRESSION EQUATION of x on y, and b is known as the
regression coefficient.
CORRELATION
56
It should be noted that while the correlation coefficient between x and y
is the same as that between y and x, and is independent of the units in
which x and y are expressed, the regression coefficient of x on y is not the
same as the regression coefficient of y on x, and neither is independent of
the units.
It sometimes happens that we wish to take account of the effect of some
third variate. For example, theoretically the pressure at sea level should
be determined almost entirely by the pressure at 9 Km. and the temperature
of the air between sea level and 9 Km. If we wish to see how far this is shown
by the observations, we must calculate the partial correlation coefficient
between pressure at sea level (x) and pressure at 9 Km. (y), eliminating the
effect of variations of temperature in the intervening air layer (z). This
partial correlation coefficient is usually denoted by rX7.z and is given by
the expression : —
The values found by W. H. Dines were : rxy = +0-68, rrz = +0-47,
ryz = +0-95. To calculate the partial coefficient accurately it would be
necessary to employ three places of decimals in the original correlation
coefficients or coefficients of zero order as they are called, but using the figures
as given we find from (6) that rI7.z = +0-85.
The result obtained shows
that when allowance is made for variations of temperature, the agreement
between pressure at sea level and at 9 Km. is much closer than when no
such allowance is made.
The regression equation of x on y and z gives the amount of variation of x
which is associated with the variation of y, z being supposed constant, plus
the amount of variation of x which is associated with the variation of z,
y being supposed constant. It thus takes the form : —
x = b^.z y + b^.y z
..... (7)
Partial correlation coefficients and regression equations between four or more
variates can be obtained by an extension of the method given above, but as
the number of variates becomes greater, the amount of arithmetic involved
increases at a very rapid rate, and it is not usually practicable to work with
more than six quantities.
Care needs to be exercised in attaching significance to correlation
coefficients. If two random and entirely unrelated series of figures are
correlated, the coefficient obtained will not usually be zero ; if a large number
of such trials be made it will be found that half of the coefficients are
numerically larger than 0-67/-y/w where n is the number of observations in
each series. Any one coefficient, therefore, has an even chance of reaching
or exceeding this value, so that with a small number of observations appreciable
coefficients may occur between unrelated series. Similarly, if between two
sets of related observations we obtain a correlation coefficient r, the most
we can say is that the true correlation coefficient, which would be given by
an infinitely large number of pairs of observations, has an even chance of
being within the range 0-67 (1 — r2)/\/w on either side of the coefficient r
actually found. This is known as the probable error of the coefficient.
Unless it is confirmed by physical reasoning or other independent evidence,
a correlation coefficient should not be accepted as significant unless it exceeds
three times its probable error, in which case the odds in favour of significance
are 20 to 1. If a number of trial correlations are made, the chance of obtaining
a single large coefficient is obviously greatly increased, and such an isolated
coefficient should not be accepted unless it is four or five times its probable
error.
57
CREPUSCULAR RAYS
If we calculate a series of values of x by means of the regression equation (5),
the standard deviation of the difference between these calculated values and
the observed values is <y/(l — rz) of the standard deviation of the latter.
Since this expression only differs appreciably from unity when r is large,
it follows that single small correlation coefficients are of little value for
forecasting purposes.
Examples of some large coefficients in meteorological work are : —
Between mean temperature of the air from 1 to 9 Km. and pressure
at 9 Km., + 0-95.
Between change of barometric pressure in 3 hours and corresponding
change in level of water in a well at Richmond, — 0 • 88.
Between the number of thunderstorms in central Siberia and the
square root of the sunspot number, + 0 • 92.
Cosmic Radiation.— It being known that air is a poor conductor of
electricity it is natural to endeavour to determine the conductivity by
utilising a closed metal vessel with an internal insulated electrode, a battery
and an electrometer. Such experiments indicate that any current through
the vessel is carried by ions which are being produced inside it and that the
rate of production of ions is governed, to a certain extent, by a penetrating
radiation from outside. The penetrating radiation diminishes at first when
the apparatus is taken up in a balloon and at greater heights it increases
rapidly. As was first pointed out by Victor Hess, this indicates that the
penetrating radiation has a component which originates outside the earth's
atmosphere. This cosmic component is far more penetrating than that from
radioactive substances, as may be demonstrated by sheathing the receiver
with many layers of lead or by sinking it in deep water. The radiation does
not come from the sun, for the intensity is independent of the time of day,
and it does not come from the stars of our galaxy, for the intensity is
independent of sidereal time. The radiation appears to be constituted by
corpuscles endowed with electric charges, for it reaches the earth in diminished
strength in the zone remote from the magnetic poles. Further, more radiation
reaches the earth from the west than from the east and this is interpreted as
a proof that a majority of the incident corpuscles are positively charged.
The corpuscles are thought to be positrons, each having the same mass as an
electron and an equal charge though of the opposite sign. As to the ultimate
nature of the cosmic radiation Prof. P. M. S. Blackett* writes : — " The
fastest atomic particles produced hitherto have energies of a few million
volt-electrons. The particles emitted spontaneously by radioactive substances
have energies of the same order of magnitude. But the particles which are
always travelling through space, and which are continually bombarding the
terrestrial atmosphere have energies which are certainly greater than
10,000 million volt-electrons and are probably greater than 100,000 million
volt-electrons. As to whence these particles come we know nothing. As to
how they are produced we know even less. That is not because we have an
embarrassing choice between several plausible theories ; it is simply because
we have not any theory. We do not even know whether these particles have
been produced by matter subject to the laws with which we are acquainted.
It may be that the cosmic rays are to be regarded as archaeological remains
dating from a time when the universe was young and very different."
Crepuscular Rays.— Occasionally soon after sunset the sky is divided up
into lighter and darker areas by lines which diverge from the position (below
the horizon) of the sun. The lighter areas indicate parts of the atmosphere
illuminated by sunshine, the darker areas those from which the sunshine is
cut off by intervening mountains or by clouds. The phenomenon is essentially
the same as that seen in the day-time when the sun is hidden by cloud and
" ladders " diverging from the sun appear where there are gaps in the cloud.
The " crepuscular rays " are coloured, the illuminated areas being pink and
* " La Radiation Cosmique " Paris, 1935.
CROP WEATHER
58
the shadows appearing greenish by contrast. The divergence of the
" crepuscular rays " is an optical illusion as light rays from the sun are
practically parallel.
Under favourable circumstances the shadows are thrown right across
the sky and to an observer with his back to the sun the rays appear to converge
to a point which is a little above the horizon. These anti-crepuscular rays
are generally ill-defined and may be mistaken for patches of cloud.
Crop Weather.—For the study of meteorological factors in the growth
and yield of crops " Crop Weather " stations were established in 1924 at
various Agricultural Colleges, Research Centres and Farm Institutes. In
general, the stations are sited in juxtaposition to growing crops upon which
measurements are made, so that the meteorological and plant observations
may be closely correlated. In addition, phonological observations are made
at certain centres. The scheme is under the control of the Meteorological
Office, the Ministry of Agriculture and Fisheries, and the Department of
Agriculture for Scotland. In recent years the scheme has been extended to
include observations of forest trees, and the Forestry Commission has
established stations at certain nurseries.
Cross Section.—As used in meteorology the term means the representation
in schematic form of conditions prevailing in the atmosphere in a vertical
plane from the surface up to any desired height along a line from one point
to another. The information plotted on the cross section depends upon the
purpose for which it is required. Cross sections for the use of aircraft pilots
flying between specified points show frontal surfaces, winds, diagrammatic
clouds, liability to icing etc. according to a conventional scheme laid down
by the International Civil Aviation Organization.
Cumulonimbus (Cb.).— Heavy masses of cloud, with great vertical development,
whose cumuliform summits rise in the form of mountains or towers, the upper
parts having a fibrous texture and often spreading out in the shape of an anvil.
The base resembles nimbostratus, and one generally notices virga. This
base has often a layer of very low ragged clouds below it (fractostratus
fractocumulus}.
Cumulonimbus clouds generally produce showers of rain qr snow and sometimes
of hail or soft hail, and often thunderstorms as well.
If the whole of the cloud cannot be seen the fall of a real shower is enough to
characterise the cloud as a cumulonimbus.
Cumulus (Cu.).— Thick clouds with vertical development ; the upper surface
is dome shaped and exhibits rounded protuberances, while the base is nearly
horizontal.
When the cloud is opposite to the sun the surfaces normal to the observer
are brighter than the edges of the protuberances. When the light comes from the
side, the clouds exhibit strong contrasts of light and shade ; against the sun,
on the other hand, they look dark with a bright edge.
True cumulus is definitely limited above and below, its surface often appears
hard and clear cut. But one may also observe a cloud resembling ragged cumulus
in which the different parts show constant change. This cloud is designated
fractocumulus (Fc.}.
Current, Ocean.—A general movement of a permanent or semi-permanent
nature of the surface water of the ocean. The term must not be used to denote
tidal streams, which change direction and velocity hour by hour. A drift
current is a drift of the surface water, which is dragged along by a wind
blowing over it. If a drift current meets an obstruction, such as a shoal or
land, it will be deflected but will be carried on by its momentum in a new
direction, forming a stream current. A stream current may be formed as a
counter or compensating current to another current, to replace the water
displaced by the primary current. The set of a current or tidal stream is
the direction in which it is going. The distance through which a current
flows in a given time is sometimes called the drift, but this term has dropped
out of use in the Royal Navy.
59
CURVE FITTING
Owing to the high specific heat of water, the main ocean currents are of
great climatic importance. Ocean currents result from several causes, the most
important being wind and differences of density resulting from differences
of temperature and salinity. The best known currents are those of the North
Atlantic. The north-east and south-east TRADE WINDS give rise to currents
directed south-westwards and north-westwards respectively, which combine
near the equator and turn westwards as the Equatorial Current. This divides
off the coast of Brazil, the greater part passing northwards by the Gulf of
Mexico and the West Indies to form the Gulf Stream which moves north­
eastward along the east coast of America at a speed of about 70 miles a day.
Off the Newfoundland Banks the Gulf Stream meets the cold Labrador Current,
which greatly lowers its temperature. At Newfoundland it breaks up,
branches going to Greenland and Iceland while the main current continues
towards Europe. About 40° N. 40° W. the latter again divides ; the southern
branch runs along the coast of north-west Africa as a cold current, while the
northern branch, under the influence of the prevailing SW. winds, traverses
the western coasts of Europe from Iceland to Norway as a warm current,
finally passing into the Arctic Ocean where it is overlain by colder but fresher
water which originates mainly in the great rivers of Siberia.
From the Arctic Ocean a cold ice-bearing current flows down the east coast
of Greenland, round Cape Farewell and up the west coast of Greenland as far
as Disco Island. Here it turns westward and then again southward as the
Labrador Current to Newfoundland. The cold but relatively fresh Labrador
Current and the warm saline Gulf Stream have nearly the same density and
flow alongside for some distance, gradually intermixing. The mixture is
slightly heavier than either of the original currents, and this produces a
" density wall," on either side of which the currents are opposed.
In the other oceans the currents are simpler. There are warm currents
resembling the Gulf Stream in the western parts of the North Pacific (the
Kuro Shiwo), the South Atlantic and less definitely the South Pacific. In the
North Pacific the warm current is unable to pass through Bering Strait but
turns south-eastward along the west coast of North America. In the South
Atlantic and South Pacific the warm currents join a great stream of water which
travels round the globe from west to east in about 50° S., picking up ice from
the Antarctic and sending branches northward along the west coasts of South
America, South Africa and Australia. In the Indian Ocean the currents are
seasonal, governed by the monsoon.
Curve Fitting.—Two main types of problems arise in connexion with
curve fitting. We may either require to represent a variable quantity as a
function of some independent variable such as time, distance, latitude, etc.,
by drawing a curve to represent the functional relationship, or we may
require to represent the frequency of occurrence of different values of a
quantity, whether measured directly or indirectly, by means of a curve whose
formula must be obtained. The simplest example of the first type is the
fitting of a straight line to represent the relationship between pairs of
measurements of two quantities, as is done in the normal treatment of
CORRELATION between two variables. When the graphical representation of
the observations indicates that the relationship is not linear, no general rule
can be laid down for the subsequent procedure. Examination of the graph
will in some cases indicate what must be the next step. If, for example, the
graph indicates a repetition of the values of one variable at equal intervals
of the other variable, there is a periodic relationship between them, and the
data can be analysed by the methods of HARMONIC ANALYSIS. If the relation­
ship is represented by a fairly smooth curve, it is frequently possible to derive
an idea of the exact nature of the curve by plotting the data on logarithmic
or semi-logarithmic paper. If the relationship is of the nature y — Ax*,
plotting on logarithmic paper will yield the values of A and n ; while if it is
of the nature of y = Aebx, plotting on semi-logarithmic paper will yield the
values of A and b.
CYCLONE
60
In fitting curves to frequency distributions, we look in vain for any
general rule. The method of least squares lays down a procedure for fitting
the closest fitting normal error curve. Pearson has also developed a general
formula :
a
y dx
b0 + b^x + & 2#»
which can be made to fit a wide variety of frequency curves by the choice
of appropriate values for a, b0, blt b z etc. For details of this type of treatment
reference should be made to Elderton's " Frequency Curves and Correlation "
(C. and E. Layton).
Cyclone.—A name given to a region of low barometric pressure. In
temperate latitudes the cyclone is now usually spoken of as a DEPRESSION
and the term " cyclone " is taken to refer only to a " tropical cyclone."
In this restricted form the term cyclone reverts to its original meaning.
Cyclonic storms are confined to very definite regions and occur on the
western sides of the great oceans. The principal regions where these storms
occur and the names by which they are known are :— (a) The Gulf of Mexico,
the West Indies and the coasts of Florida where they are known as
HURRICANES ; (b) the Arabian Sea and the Bay of Bengal (cyclones) ; (c) the
China Sea and the coasts of Japan (TYPHOONS) ; (d) the Indian Ocean to the
east of Mauritius and Madagascar (cyclones), and (e) the Pacific Ocean to the
east of Australia and Samoa (hurricanes). In the Philippines they are known
as BAGUIOS. The storms usually originate over the ocean and their paths
generally keep to the ocean ; if the path crosses from the ocean to the land the
storms soon die out and they quickly lose their destructiveness.
The cyclone and the depression are in essentials alike ; their winds in
the northern hemisphere circulate in a counter-clockwise direction and in
the southern hemisphere clockwise, the only difference being that of their
strength. Winds of force 12 on the BEAUFORT SCALE of wind force frequently
accompany the cyclone while in the depression winds of this strength are rare.
This difference is due largely to the difference in pressure gradient.
At the centre of a cyclone the pressure is frequently about 960 mb. and
in its outer edge about 1,020 mb. The diameter of such a storm may reach
600 miles, but it is more often much less. In a depression similar differences
of pressure occur between the centre and the outer edge, but the diameter
of the depression may be 1,000 or even 2,000 miles.
In the centre of a cyclone there is usually a very limited region, only a
few square miles, in which there is a complete calm with only a narrow
strip of moderate winds separating it from the winds of hurricane force.
This calm region is recognised as the EYE of the cyclone. In it the weather
is usually fine while in the other parts of the cyclone there are cloudy skies
and torrential rain. Frequently before a cyclone approaches the weather
is fine and quiet, but the skies soon cloud over, the barometer falls quickly
and the air becomes oppressive and sultry, the wind freshens rapidly and
soon heavy rain commences. After the passage of the storm the weather
quickly resumes its peaceful form.
The velocity of translation of the cyclone is usually under 15 miles per
hour ; the velocity varies in different cyclones as in depressions in temperate
latitudes, but the velocity of translation is generally less than that of the
depression.
Cyclones are of a seasonal nature, they occur principally towards the end
of the hot seasons. In the West Indies August to October are the months
of greatest frequency. In the Southern Pacific and in the South Indian
Ocean they occur most frequently between December and March, in the
Bay of Bengal and in the Arabian Sea between April and June and again
between September and December. In the China Sea and the Antilles they
occur between July and October. These are the principal periods for the
occurrence of cyclones, but they may occur also in the months adjacent to
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CYCLONIC RAIN
62
Near the centre of a cyclone the fall of pressure is often very rapid.
Fig. 16 shows a reproduction of a barogram during a cyclone which
passed over Cocos Island (Keeling Is.), on November 27, 1909. It is
interesting to notice that in spite of the rapid fall of pressure with the onset
of the cyclone the diurnal variation of the barometer is still apparent and it
reappears before the normal level is recovered.
The lowest known pressure in the centre of a cyclone is 886-8 mb., reduced
to mean sea level; it was recorded about 400 nautical miles east of Luzon
(Philippines) on August 18, 1927.
Cyclonic Rain.—The rainfall associated with the passage of atmospheric
low pressure systems and due to the vertical motion set up by an air mass
rising over another air mass of different temperature. See DEPRESSION,
CONVECTIONAL RAIN, GEOGRAPHIC RAIN.
Cyclostrophic.—See GRADIENT WIND.
Damp Air —As distinguished from dry air, damp air implies in meteorology
a high degree of RELATIVE HUMIDITY. When its relative humidity equals or
exceeds 85 per cent, of saturation air may fairly be called damp. It will
deposit some of its moisture in dry woollen fabrics, cordage or other fibrous
material, though its water will not condense upon an exposed surface until
100 per cent, is reached.
Dawn.—(O.K. daglan, to become day), the time when light appears (daws)
in the sky in the morning or the interval between the first appearance of light
and the rising of the sun. See TWILIGHT.
Day.—A solar day is the interval of time which elapses between successive
occasions on which the sun is in the meridian of any fixed place. These
occasions are known as " transits " of the sun. A sidereal day is the corres­
ponding interval between successive transits of a distant fixed star. Since
the earth moves in an orbit round the sun in the same sense of rotation as
that of its rotation about its axis, the length of the solar day is slightly
greater than that of the sidereal day.
Owing to the eccentricity of the earth's orbit and to the inclination of
the equator to the ecliptic, solar days are of slightly unequal lengths at
different times of the year. The average length of the solar day is 86,400
seconds. This is called the " mean solar day " and it is taken as the length
of the civil day, or, the " day " of ordinary parlance. The sidereal day contains
86,164 seconds nearly.
The epoch of the beginning and end of the civil day is fixed by the civil
power. In this country a civil day in winter begins and ends at midnight
Greenwich Mean Time ; in other words, the standard longitude for purposes
of civil time reckoning is that of Greenwich. During the operation of
BRITISH SUMMER TIME, however, the civil day begins and ends at 23 h. G.M.T.
Debacle.—The breaking up in the spring of the ice in the rivers. The
term is chiefly applied to the great rivers of Russia and Siberia and of the
North American continent. Debacle lasts from two to six weeks ; during
the period the rivers often overflow their banks, inundating the surrounding
country. In southern Russia debacle begins about the middle of March, in
latitude 55°- 60° N., it begins early in April, but in the north it does not
begin until May and in the extreme north of Siberia not until June. In
Canada in Ontario the debacle takes place in March and the water is free by
April, in the St. Lawrence it is a little later, the river being free of ice in May.
Declination, Magnetic.—The angle between the magnetic meridian (the
direction of the magnetic axis of a freely suspended or pivoted magnetic
" needle ") and the geographical meridian. This angle is sometimes confusingly
referred to as the " magnetic variation." Declination is subject to a regular
diurnal variation, to irregular comparatively short-period changes, and to
secular change. See TERRESTRIAL MAGNETISM
63
DENSITY
Dekad.—In meteorology, a period of ten days, but decade is often used
or ten years.
Density.—The mass of unit volume of a substance. The density of pure
water, for example, is 1-000 gram* per cubic centimetre at a temperature
of 277°A. It is necessary to specify the temperature in stating the density
as at any other temperature the same sample of water will occupy a greater
volume, so that the mass per cubic centimetre is then less. At 283°A. the
density is 0-998 gm./cm3. Strictly, the pressure also should be stated, as
the volume of a sample of water alters with the pressure to which it is sub­
jected ; but for ordinary pressure changes these alterations in the case of a
liquid or of a solid are so slight as to be of no practical consequence.
In meteorology we are concerned more particularly with the density of
the air, and it is necessary to specify not only the temperature and pressure
when stating a numerical value for it, but also its constitution. Dry air is a
mixture of gases of which oxygen and nitrogen form about 99 per cent.,
the remainder consisting of carbon dioxide and a number of rare gases of
which argon is the chief (see ATMOSPHERE). For all practical purposes
the relative proportions of these gases in the atmosphere remain constant,
so that dry air may be regarded as a single gas when considering questions
of density ; but atmospheric air is never completely dry, as it always contains
a quantity of water vapour which, though relatively small, is variable and,
in consequence, causes appreciable changes in the density.
The factors which affect air density are, therefore, pressure, temperature
and the proportion of water vapour present. An increase of pressure, if
temperature and water vapour content remain unaltered, will increase the
density. An increase of temperature, if pressure and water vapour content
remain the same, will result in a decrease in the density. If the proportion
of water vapour is increased, temperature and pressure remaining the same,
the density will be diminished ; the pressure in this case is the sum of the
air pressure and the pressure exerted by the water vapour, so that for the
total pressure to remain unaltered the addition of water vapour, which exerts
its own pressure, must be made by displacing some of the air. As the density
of water vapour is less than that of air at the same pressure the average
density of the mixture after the introduction of more water vapour is lower
than the original density. A more usual feature of the practical problem is
perhaps the loss of water vapour by condensation. If the original sample
of air is saturated a fall of temperature will result in an increase of density
if pressure remains the same, not merely on account of the direct effect of
the lower temperature alone, but also by the lowering of the proportion of
water vapour present in the sample, following the condensation which results
from the fall of the temperature below the dew point.
The effect of each of these factors is quite definite and has been determined
by measurements. These measurements result in the following formula for
the calculation of the density of any given sample of air, or the density of
atmospheric air, when the temperature, pressure, and pressure of the aqueous
vapour present is known :—
J = A. t^i-' . 5
T
Po
where
A is the density of the sample of air to be computed,
zJ0 is the density of dry air at pressure p0 and temperature T0, absolute,
p is the barometric pressure of the sample,
T is the temperature, in degrees absolute, of the sample,
e is the pressure of aqueous vapour in the sample.
* The authors of the metric system intended the relation to be precise, but it is nowaccepted that the maximum density of water is -999972 gram per cubic centimetre.
DEPRESSION
64
The density of dry air at a pressure of 1000 mb. and a temperature of 290°A.
is 1,201 grams per cubic metre, so that the formula may be written :—
A = 1201 . P~ % e . ???
1000
T
p and e being measured in millibars.
The numerical value of e is readily determined from the readings of a
dry- and wet-bulb psychrometer, from which the vapour pressure for the
temperature of the sample, that is, the dry-bulb reading, is obtained by
reference to suitable hygrometric tables.
The question of air density is of importance in meteorology, as it is
because of the different densities of adjacent masses or converging currents
of air that convection takes place in the atmosphere ; the less dense air
rising while that with the greater density tends to descend. In this way
vertical circulation, as seen in such phenomena as land and sea breezes,
is brought about. The rain and cloud in certain parts of low-pressure systems,
or depressions, are also due to the rising of air of comparatively low density
over currents of denser and usually cooler air. KATABATIC winds may also
be mentioned as important meteorological phenomena due primarily to a
difference between the density of the air on the slopes of a hill and that of
the air at the same level above the valley.
Atmospheric density varies with both time and place. At a height of
about eight kilometres, however, the density is uniform. Above this height
the density at any level decreases as we go from the equator to either pole,
while below this height the density increases from equator to pole. At a
height of eight kilometres, also, the density does not change appreciably
throughout the year. Above this height density is normally greatest in
summer, while below this level density is at its maximum in winter.*
Normally density is approximately uniform in a horizontal plane, but
this does not hold where local circulations exist. For different types of
pressure distribution the late W. H. Dines has given the following values
for the density at the heights stated over the British Isles. The relative
humidity is assumed to be 75 per cent.
Height
Anticyclone
Depression
Height
Anticyclone
Depression
Km.
10
9
8
7
6
gm./ms.
421
474
531
595
662
gm./m'.
382
444
514
583
652
Km.
5
4
3
2
1
gm./ms.
736
818
911
1,012
1,137
gm./ms.
724
807
893
992
1,100
Depression.—A depression is a part of the atmosphere where the barometer
is lower than in the surrounding parts. It is occasionally called a " cyclone "
or simply a " low." The isobars round such an area of low pressure are more
or less circular or oval. Depressions vary enormously in size, one may be
only a hundred miles in diameter and another over two thousand miles ;
some are deeper than others, a deep depression being one in which the pressure
is very much lower near the centre than on the outside while, on the other
hand, a shallow depression is one where the pressure, although perhaps low
near the centre, is not very much lower than in the surrounding districts.
North of the equator the wind blows round the depression in a counter­
clockwise direction, with some motion inwards across the isobars ; its strength
is in all cases closely related to the steepness of the barometric gradient, the
steeper the gradient the stronger the wind. The weather in a depression is of
* S. N. Sen.: " On the distribution of air density over the globe."
met. Soc., 50, 1924, pp. 29-51.
London, Quart. J.R.
65
DEPRESSION
an unsettled type. Depressions in middle latitudes usually move in an east
to north-east direction but any direction may be followed. The velocity of
translation varies with each depression, and in any single one the velocity
is never constant. Some depressions may move as much as 600 or 700 miles
per day, while others remain practically stationary. Depressions, in their
movement, carry their weather with them, but the weather is subject to
changes due to changes which take place in the depression itself.
The origin of a depression has formed an interesting study to many
investigators. It used to be thought that some local cause of heating of
the air led to a rising column of warm air which owing to its reduced weight
gave low pressure at the base. A wind circulation was set up around the
area of low pressure and the depression was formed. This theory was generally
held until upper air observations showed that in the free air depressions tend,
on the average, to be cold not warm, and that at the centre of a well-marked
depression, pressure is low not only on the surface of the earth but at the
base of the STRATOSPHERE, which in well-marked depressions is found some
five miles above the surface. This suggested that depressions were not
phenomena of the lower atmosphere only, and that it might be necessary
to seek their cause either in the layers above a height of five miles which
form the stratosphere or at the surface of separation between the lower layers,
the TROPOSPHERE and the upper layers, the stratosphere. The impossibility
of obtaining a network of day-to-day observations in the stratosphere
hampered and still hampers investigation on these lines, and in the last
twenty years attention has again been concentrated on the troposphere by the
ingenious theories put forward by V. and J.Bjerknes, father and son, of Bergen
and by Professor Exner, of Vienna. Both these theories ascribe depressions
to the interaction between warm and cold air masses, and as the Bjerknes
theory has been widely adopted among European meteorologists, a brief
account of it will be given. It is based on the observed fact that most
depressions originate on a comparatively straight FRONT. In the simplest
case, which is illustrated in Fig. 17 (a), the cold and warm currents are flowing
in opposite directions. In all cases there is a wind discontinuity at the front
of cyclonic type (i.e. if the directions are the same, the stronger wind is on
the side of the higher pressure). From the front a SURFACE OF DISCONTINUITY
slopes upwards over the cold air. The stability of the system pictured is easily
disturbed, and waves develop on the surface of separation which are shown
on the earth by tongues of warm air projecting into the polar air and deflect­
ing the polar front as in Fig. 17 (b). Under these conditions the currents no
longer flow parallel and side by side. The warm air current climbs over the
colder air in front, while the cold air in the rear undercuts the warm air.
Pressure falls at the tip of the tongue of warm air and the isobars begin to
assume the form associated with depressions. The line where warm air is
climbing over cold air is termed the WARM FRONT of the depression, the line
where cold air undercuts warm, the COLD FRONT. The ascent of the warm
air tends to narrow down the tongue of warm air and ultimately the warm
air is driven from the surface altogether, the cold air in the rear catching up
the cold air in the front. The depression is then said to be occluded (see
OCCLUSION), and soon begins to decrease in intensity. The portion of the
front AB in Fig. 17 (c) is an occlusion. Such in brief outline is the life history
of a depression as viewed by the Bergen School. The rainfall, which forms
such a notable feature of most depressions, is chiefly confined to the region
along the fronts. Warm-front rain falls ahead of the line where warm air is
rising over cold, and cold-front rain, which often takes the form of short heavy
showers accompanied by squalls, falls where the cold air is undercutting
the warm.
It may be asked why, if depressions can be divided into warm and cold
sectors in this way, synoptic charts had been drawn daily for some 40 or
50 years before meteorologists discovered the fact. The answer undoubtedly
is that depressions generally reach Europe from the Atlantic and are nearly
(90565)
C
DEPRESSION
66
always occluded before entering the land area where detailed analysis of the
conditions becomes possible. It is thus rare to find well-marked warm sectors
with their attendant warm and cold fronts on the daily weather maps prepared
by the European meteorological services, and only by careful study and expert
knowledge can the position of the fronts be located.
Cold air
FIG. 17.
It may be pointed out that at times, depressions appear on the weathe
map which seem to have formed entirely within polar air, and it may ber
necessary to return to a modified form of the older local heating theory.
would seem to follow, therefore, that depressions not only differ amongIt
st
themselves but also differ in their origin.
The life of a depression is very varied and may extend from one or two
to five or more days. When a low pressure system appears to maintain its
existence for a long period it frequently happens that it is reinforced from
time to time by combining with associated SECONDARY DEPRESSIONS which
have developed on its southern side. (See Plate III).
to face Page 66
Plate JE
HIGH
/r-
H
MARCH 30, 1928, 18 h.
DEPRESSION
67
DEWPOND
Desert.—A region in which the high temperature and small rainfall
cause the evaporation to exceed the precipitation, and consequently there is
insufficient moisture to support vegetation. Deserts may be caused by any
of the following conditions : (1) the presence of a persistent anticyclone, an
example due to this cause occurs in northern Africa where the Sahara coincides
with the average position of the sub-tropical belt of high pressure ; (2) a cold
current on the western coast of a large mass of land such as occurs in Peru ;
and (3) a configuration of ground which shuts out the moisture-bearing winds,
for example, Gobi, in central Asia.
For a formula used by Koppen and Geiger for the limit of rainfall which
constitutes a desert climate, see ARID.
Desiccation.—The permanent disappearance of water from an area due
to a change of climate and especially a decrease of rainfall. Large areas
in central Asia, Africa and western America have been desiccated since the
Glacial period, but there does not appear to have been much progressive
desiccation during the past 2,000 years.
Deviation.—The difference of an observation from the mean value of the
series of which it forms a part. For example, the mean January pressure at
Kew, over the period 1901-30 is 1016-7 mb., the mean in January, 1938,
was 1012-3 mb., and the deviation —4-4 mb. The deviations of a series of
observations can be summarised by finding either the mean deviation or the
STANDARD DEVIATION. The mean deviation is the arithmetic mean of the
individual deviations taken without regard to sign ; the standard deviation
is the square root of the arithmetic mean of the squares of the individual
deviations. In most series of observations the standard deviation is about
1J times the mean deviation.
The angle between the surface WIND and the direction of the isobars is
sometimes termed the deviation of the wind.
Dew.—Water drops deposited by condensation of water vapour from the
air, mainly on horizontal surfaces cooled by nocturnal radiation. In the case
of grass and the foliage of plants the greater part of the water drops which
appear after a cold night were proved by Aitken to be the result of exudation
of water from the plant.
Dew Point.—The temperature to which air can be cooled without causing
CONDENSATION. It is the temperature for which the saturation vapour
pressure is identical with the pressure of the vapour in the air.
Dewpond.—A pond on high ground on chalk downs which retains its
water during all but the most prolonged droughts, after ponds at lower
levels are dried up. A dewpond is artificially constructed, a watertight
bottom being made of about 9 in. of puddled chalk or clay treated with
lime ; a layer of chalk rubble is placed over the bottom to protect it from
the feet of cattle. Sometimes a layer of straw is placed below the water­
tight bottom, sometimes it is placed above—between the bottom and the
chalk rubble ; in a few ponds there are two or three alternate layers of straw
and puddled clay, in others there is no layer of straw at all.
The chief interest in dewponds lies in the mystery of their supply of
water ; they do not dry up in hot summer weather in spite of the watering
of cattle. The idea that gave the ponds their name was that they were
replenished at night by dew, the theory being that the non-conducting layer
of straw prevented the outward radiation from the heated earth from reaching
the water, and that the water, therefore, was kept cool and dew condensed
on it. Careful experiment, however, seems to show that only very rarely
in the short summer night does the temperature of the water fall below the
dew-point. Another suggestion was that the water was replenished by the
heavy night mists which enshroud the higher part of the downs during hot
summer weather, the mist condensing on surrounding shrubs and dripping
(90565)
C2
DIATHERMANCY
68
into the pond or being deposited mechanically by turbulence. In any case
by far the greater part of the supply of water must come from rain, which is
greater at high levels than in the plains, the wide shelving margins of the
ponds serving as GATHERING GROUNDS.
Diathermancy—Diathermanous.—(Based on B. Stewart. " Treatise on
Heat." 2nd Ed., pp. 183-5.) The discovery, by Melloni, that rock salt is a body
which transmits heat freely, sufficiently indicates that most substances which
are transparent for light are not so with regard to heat. This is expressed
by saying that most substances are " athermanous " and that rock salt is
almost the only " diathermanous " solid, these two words corresponding to
the terms opaque and transparent in the science of optics. Tyndall found
that a solution of iodine in bisulphide of carbon has the property of completely
stopping the light rays, while it allows dark heat to pass in great quantity.
He found that a fluid lens formed of this solution and enclosed in rock salt
will stop all the light from an electric lamp but permit the dark rays to pass
in abundance.
Oxygen and nitrogen are diathermanous, water vapour and carbon dioxide
absorb heat rays of certain wave-lengths so that atmospheric air is only
partially diathermanous.
Diffraction.—Ordinary experience with light suggests that it always passes
through the air in straight lines, and that it cannot go round an obstacle.
This is an apparent contrast with sound ; for we hear without difficulty round
a corner. There are, however, numerous phenomena which indicate that light
can be diverted from the straight course by obstacles. These phenomena
are grouped under the name of diffraction. They can be explained by the
wave theory of light. According to this theory the contrast between the
behaviour of light and sound is a matter of scale, the wave-lengths of light
being very small in comparison with those of sound.
Light is diffracted by waterdrops to produce CORONAE, as well as other
colour effects observed on clouds. A rarer phenomenon, BISHOP'S RING, is
explained by diffraction by dust particles in the upper atmosphere. The
blue of the sky is due to the scattering of light by the molecules of which the
air is constituted. Scattering of this type is regarded by some authors as
covered by the term diffraction.
Diffusion.—The term diffusion is used in meteorology to indicate either
molecular diffusion or else eddy diffusion.
Molecular diffusion is the process by which contiguous fluids mix slowly
even in spite of differences in their density. The process of molecular diffusion
follows certain definite laws which are similar to those for the diffusion of
heat by thermal conduction. This process is so slow, however, that in practice
the effects due to it are usually negligible compared with those produced by
eddy diffusion.
Prof. G. I. Taylor has shown that eddy diffusion, or mixing in the atmos­
phere by turbulent motion (see TURBULENCE) follows the same type of law
as molecular diffusion in certain respects. More recent work, particularly
that of L. F. Richardson, has shown that this similarity or analogy extends
only up to a point. The laws relating to eddy diffusion appear to be more
complex than those for the better-known type of diffusion. This difference
is due, at least partially, to the fact that the eddies which cause the diffusion
vary over a wide range of size.
Discontinuity.—As a rule the fundamental atmospheric variables, pressure,
wind velocity, temperature and humidity, are continuous functions of space
and time. Occasionally, however, their variation in a small distance (or in a
short time at a fixed point) is so much above normal that their distribution
is regarded as discontinuous. For example, a fall of 10° F. in temperature.
69
DOCTOR
usually spread over some hours at least, may take place in a few minutes and
this indicates what may be regarded as a discontinuity in the atmosphere.
See also SURFACE OF DISCONTINUITY.
Disturbance.—Sometimes used instead of DEPRESSION, especially in broad­
cast general inferences.
Diurnal Variation.—This term is used to indicate the changes in the course
of an average day of the magnitude of a meteorological element. If we have
a series of hourly values of the element for a large number of days, and
if we determine the average values for those days of the element at lh.,
2h., .... 24h., it will be found that the averages show steady and regular
variations during the course of the twenty-four hours, and that irregular
fluctuations which may appear on any of the days are eliminated from the
averages provided the number of days taken is large enough. Thus the
characteristic feature of the diurnal variation of atmospheric pressure is a
12-hourly oscillation which has its maxima at the same local time all over
the intertropical and temperate zones. In the polar regions the oscillation
has its maxima at the same absolute time along a circle of latitude. The
amplitudes of those waves are greatest at the equator and diminish towards
the poles. This double wave of pressure may be seen occurring with great
regularity each day on the traces from a self-recording barometer in the
tropics, the maxima occurring approximately at lOh. and 22h. and the minima
at 4h. and 16h. In the British Isles non-periodic changes in pressure, due to
the passage of cyclones and anticyclones, are as a rule so relatively large that
they obliterate the 12-hourly oscillations, but the latter may be recognised
in periods of quiet settled weather, and are evident when averages are taken
of hourly values as explained above. Other elements showing conspicuous
diurnal variations, especially during the warmer months of the year, are air
temperature and relative humidity. Diagrams showing the diurnal variation
in each month of air temperature, based on data for 25 years at Aberdeen,
Kew Observatory, Valentia Observatory and Falmouth are given in " Tem­
perature Tables of the British Isles " (M.O. 154, 1902). Temperature is
normally at a maximum about two hours (in winter) to three hours (in
summer) after local mean noon and at a minimum about sunrise. Relative
humidity depends not only on the amount of moisture which the air contains,
but on the temperature of the air, and on days when the temperature range
is large relative humidity also exhibits large variations ; it is normally at a
minimum in the afternoon about the time of occurrence of maximum tem­
perature, and at a maximum in the early morning about the time of occurrence
of minimum temperature. The Observatories' Year Book, a serial publication
of the Meteorological Office, contains many data relating to the diurnal
variations of meteorological and geophysical elements.
Divergence.—Divergence is the opposite of CONVERGENCE, the article on
which should be consulted. If the winds are such that more air flows out of
a given area than flows into it there is said to be divergence. The deficiency
of air must be made good by a downward current from the upper layers.
Such a downward current is termed SUBSIDENCE. The chief effects are absence
of precipitation and the frequent development of INVERSIONS with dry air
above them. Since the lower layers of the atmosphere hold most moisture,
divergence tends to cause a steady decrease in the total water content in the
column of air over a given area. No appreciable divergence can take place
with a system of geostrophic winds (see GRADIENT WIND). The two chief
causes are, first, surface friction, which causes a flow of air near the ground
from high to low pressure, with divergence from an anticyclone or ridge of
high pressure. Secondly, there is divergence from a region of rising barometer,
owing to lack of perfect balance between wind and pressure gradient. This
effect is smaller near the ground than the frictional divergence from an anti­
cyclone, but it extends much higher and is probably often more important.
Doctor.—See HARMATTAN.
DOLDRUMS
70
Doldrums.—The equatorial oceanic regions of calms and light variable
winds accompanied by heavy rains, thunderstorms and squalls. These belts
are variable in position and extent, and as a whole move north and south
with the annual changes of the sun's declination. The movement is consider­
ably less than that of the sun and is of the order of 5° on either side of the
mean position, with a lag of from one to two months behind the sun.
Drainage Area.—The area whose surface directs water towards a stream
above a given point on that stream. See also CATCHMENT AREA and
GATHERING GROUND.
Drizzle.—Precipitation in which the drops are very small. If the drops
are of appreciable size although the rain is small in amount the term adopted
is " slight rain."
Drops.—See RAINDROPS.
Drosometer.—An instrument for measuring the quantity of dew deposited.
A self-recording type (" drosograph") used in Italy consists of a light
horizontal pan to receive the dew the weight of which is made to actuate the
recording pen, through a system of levers.
Drought.—Dryness due to lack of rain. Certain definitions have been
adopted in order to obtain comparable statistical information on the subject
of droughts. Thus an absolute drought is a period of at least 15 consecutive
days, to none of which is credited 0-01 in. of rain or more. A partial drought
is a period of at least 29 consecutive days, the mean daily rainfall of which
does not exceed 0 • 01 in. A dry spell is a period of at least 15 consecutive days
to none of which is credited 0-04 in. of rain or more. During the 62 years
1858-1919 there were 69 absolute droughts and 163 dry spells at Camden
Square, London. The definitions of absolute drought and partial drought
were introduced in British Rainfall, 1887, p. 21, while that of dry spell was
first used in British Rainfall, 1919, p. 15. A chapter is devoted to the subject
in each volume of British Rainfall.
Dry Air.—When a meteorologist speaks of " dry air "he does not normally
mean air that contains no AQUEOUS VAPOUR, although a chemist or physicist,
speaking of laboratory experiments, would generally use the words in that
sense : the atmosphere, even in the driest parts of the world, always contains
some aqueous vapour, but in the laboratory air can be made perfectly dry,
hence the distinction. In meteorology we may take it that air referred to
as " dry " has at least a sufficiently low RELATIVE HUMIDITY for EVAPORATION
to be taking place actively from earth, rock, etc., as well as from vegetation.
In the BEAUFORT NOTATION a more precise meaning has been attached to the
words, which are applied only to air with a relative humidity of less than
60 per cent. ; the Beaufort letter for such air is " y."
Dry- and Wet-Bulb Hygrometer.—See PSYCHROMETER.
Dry Season.—A period of a month or more with little or no rain, which
recurs regularly every year. Thus on the north coast of Africa summer is
the dry season, while on the coast of West Africa the dry season occurs in
winter of the northern hemisphere. See also CLIMATIC ZONES.
Dry Spell.—See DROUGHT.
Duration of Sunshine.—The number of hours in any period (e.g. a day,
month or year), estimated according to long-established practice from the
records of a SUNSHINE RECORDER, during which the sun was sufficiently
intense to scorch the standard card when concentrated by the standard
glass sphere. Normal values of mean daily and monthly duration of sunshine
for numerous stations are given in "Averages of Bright Sunshine for the British
Isles"'
71
DUSTSTORM
Dusk.—The time when civil twilight ends, the darker stage of twilight
or the interval between the time of ending of civil twilight and the onset
of complete darkness. See TWILIGHT.
Dust.—The atmosphere carries in suspension, often for long distances,
solid particles of varying character and size. The chief sources of these
particles are volcanic eruptions, meteors, sand raised by storms of wind
over deserts and, in the neighbourhood of towns, industrial and domestic
smoke. This atmospheric dust is of considerable importance meteorologically.
If present in sufficient quantity it weakens the sunlight, this usually occurs
in calm weather in large cities.
Volcanic eruptions of an explosive type, such as that of Krakatoa in
1883, project into the higher levels of the atmosphere enormous quantities
of very fine dust, so fine that it takes more than a year to settle. The volume
of dust from Krakatoa has been estimated as 18 cubic kilometres. This
dust interferes with the passage of the sun's radiation through the atmosphere,
and the measurements made at the Smithsonian Astrophysical Observatory
on Mount Wilson after the eruption of Mt. Katmai, Alaska, in 1912, showed
a decrease of 20 per cent. Terrestrial radiation to space is also intercepted
and partly returned to the earth, but W. J. Humphreys has shown that this
process is less effective than the interception of solar radiation, so that volcanic
dust lowers the temperature. Humphreys suggests that in this way it may
be one of the causes of ICE AGES. Another effect of volcanic dust is the unequal
diffraction of light rays, producing brilliant sunsets. See SUNRISE AND
SUNSET COLOURS.
Whether ordinary dust particles act as nuclei for the condensation of
water vapour in the air is not clearly understood. There are present in the
atmosphere, as well, particles of hygroscopic substances such as common
salt, sulphuric acid, etc. ; these are known as " hygroscopic nuclei," and
molecules of water will condense on them when the air is only 75 per cent,
saturated.
The amount of dust present in the air can be measured by a DUST
COUNTER.
Dust Counter.—An instrument for counting the dust particles in a known
volume of air. In Aitken's dust counter condensation of water is made to
occur on the nuclei present and the number of drops is ascertained.
Hygroscopic nuclei as well as true dust particles are, therefore, recorded.
In Owens' dust counter a jet of damp air is forced through a narrow slit in
front of a microscope cover glass. The fall of pressure due to expansion of
the air after passing the slit causes the formation of a film of moisture on the
glass, to which the dust adheres, forming a record which can be studied under
a microscope.
Dust Devil.—A whirlwind, formed by strong convection over a dry sandy
region, which carries up the dust into the air with it. Dust devils have been
observed to rotate both in clockwise and counter-clockwise directions. Their
estimated heights vary considerably, in some cases they are said to reach
as high as 2,000 or 3,000 ft. The speed with which they move also varies very
much, it may be under 5 miles an hour, or it may exceed 30 miles an hour.
Duststorm.—Duststorms occur when a very turbulent wind passes over
dry sandy or dusty soil. The winds associated with them may be relatively
strong but not necessarily very strong, their most pronounced characteristic
being their turbulent nature rather than then: strength. Their arrival is
usually marked by the advance of a "wall of dust," towering upwards for
several thousand feet and sometimes extending up to over 10,000 ft. In
advance of this wall the wind is light and the weather oppressively warm,
while behind it, in the duststorm, the wind rises and visibility is reduced to
low limits by the flying dust and debris raised from the ground. These storms
DYNAMIC COOLING
72
are frequent in such arid desert regions as are to be found in the middle of
north Africa and in the plains of Iraq, north-west India and the United
States.
For the development of a duststorm there are three essentials : (1) an
ample supply of fine dust or dry silt, (2) relatively strong winds to stir it up
and (3) a steep lapse rate of temperature in the dust-carrying air. The last
two conditions are frequently associated with the thundery squall which
blows outwards in front of an advancing thunderstorm. For this reason
duststorms are particularly liable to occur in thundery weather in desert
regions. In such regions the air is frequently so dry that the rain associated
with the thunderstorms evaporates before it reaches the ground, but when
the rain does reach the ground the dust is washed down with it and the duststorm is usually short-lived and patchy.
There is a marked distinction between duststorms and sandstorms. See
SANDSTORM.
Dynamic Cooling.—The fall of temperature produced by expansion due
to diminished pressure. See ADIABATIC.
Dyne.—The C.G.S. unit of force. It is the force which, applied to a mass
of one gramme during one second will produce a velocity of one centimetre
per second.
Earth Currents.—The existence of electric currents flowing in the earth's
crust and the association of abnormally high values of such currents with
magnetic storms and notable auroral displays have been known since the
early days of telegraphy. Apart from observations of the current in ordinary
telegraphic lines, systematic information is obtained by recording continuously
the potential difference between a pair of similar electrodes buried in the
earth, under as nearly as possible like conditions, and separated by a
convenient distance which should not be much less than a mile. In order to
obtain data as to the resultant current, or potential difference, it is desirable
to have two pairs of electrodes situated respectively in and perpendicular to
the meridian. Because of lack, until recently, of reliable data as to the
conductivity of the earth's crust in the neighbourhood of recording stations,
it is customary to express the observational results not in terms of current
density but in terms of potential gradient between the pairs of electrodes.
Owing to various inherent difficulties (electrode effects, local peculiarities,
etc.), there is considerable uncertainty as to average values and resultant
direction of the earth potential gradient. It appears that in normally
quiet times the average potential gradient may be as high as 0-2 or 0-3
volt/Km., although the values found in several places are very considerably
less than this ; while at times of disturbance values of at least 1 volt/Km,
have been observed. Data as to the general direction of flow of earth currents
are somewhat confusing, for example, 17° E. of N. at Haparanda, 59° at Lund,
19° at Houlton, 25° at Wyanet, 31° at New York, 70° at Tucson, 79° at
Chesterfield, 63° at Fairbanks (Alaska) and 80° at Watheroo (Western
Australia). It is considered that the distribution of land and water exerts
an important influence on the whole system of superficial currents.
Diurnal Variation.—The earth potential gradient undergoes regular solar
and lunar diurnal variations, the lunar variation having about one-fifth the
range of the solar.
Considerable differences in type and amplitude are found between the
solar variations at different stations. At most places two maxima and minima
exist, the chief minimum of the north component occurring near noon in the
northern hemisphere, but at Huancayo (Peru) the variation is a simple
oscillation centred at noon. The variation at Watheroo is similar in form but
opposite in sign to that in the northern hemisphere. Except at Tucson,
Arizona (where the variations are parallel) the eastward component varies
similarly to the northward, but is opposite in phase. The range of the solar
diurnal variation is least in winter and greatest in spring and summer.
73
EARTHQUAKE
Connexion with Terrestrial Magnetism.—Part, at least, of the electric field
in the earth's crust is connected with that associated with the magnetic field.
The primary electric current system considered as existing in the ionosphere
produces during magnetic storms a secondary field in the earth (at a depth of
some 250 Km. according to Chapman), with currents also in the surface
strata ; in such cases and only in such cases is there a close parallelism
between recorded earth currents and magnetic variations. In magnetically
quieter conditions there is similarity between the east component of the
earth current and the horizontal component of magnetic force, but a pro­
nounced difference between the north component of the earth current and the
east component of magnetic force.
Earthquake.—Any rapid movement of the ground may be called an
earthquake. The term is usually restricted, however, to movements which
originate naturally and below the surface. There is an enormous range in
the intensity of earthquakes. In a destructive earthquake the range of
oscillation of the ground may be as much as four inches, and the maximum
acceleration of the movement may be comparable with the acceleration due
to gravity. Permanent changes of level may occur. Such a great earthquake
may be felt by sensitive persons 1,000 miles from the central area.
The initial displacement which results in an earthquake usually occurs
at some considerable depth below the surface. The place where this initial
displacement occurs is called the focus or hypocentre. The spot on the
ground vertically above the focus called the epicentre. The depth of the
focus is only a mile or so for volcanic earthquakes. The small earthquakes
experienced in the British Isles (e.g., Hereford and Jersey, 1926) have focal
depths of less than 6 miles.* For greater earthquakes the depth is generally
comparable with 30 miles, but in certain regions, notably round the Pacific
Ocean, there are " deep focus earthquakes " which originate at depths from
100 to 350 miles.f
Each year there are on the average about 5,000 earthquakes large enough
to be felt.J Of these about 100 are more or less destructive. The number
of earthquakes the waves from which are registered by seismographs in
England is about 300. Nearly half of these have their origins under the sea.
The larger earthquakes are all to be attributed to the yielding of the
rocks under the influence of such forces as produce the folding of strata and
the growth of mountains. Probably the fracture usually occurs along an
existing fault, one part of the rock shearing over another. In some cases
the shearing movement extends to the surface ; the amount of the dis­
placement may be as much as 20 ft., vertically or horizontally. The dislocation
which takes place in a single earthquake may not relieve the strain entirely.
A great earthquake is usually followed by a series of after-shocks.
To the question whether the frequency of earthquakes depends on the
influence of the moon or on the weather no very definite answer can be
given except that such influences are by no means dominant.
The waves which are propagated from an earthquake and are recorded
by seismographs pass through the body of the earth or round the crust.
The first to reach a distant station are body-waves of compression and
expansion, these are followed by body-waves of distortion and these again
by the surface waves. Thus when the distance from the epicentre is onequarter of the earth's circumference the times of passage are approximately
13 min. 16 sec., 24 min. 14 sec. and 43 min. 30 sec. for the three types of
wave. The superficial waves are generally much larger than the body-waves,
which together constitute the " preliminary tremors," but deep focus earth­
quakes produce comparatively small superficial waves. The discussion of the
behaviour of the different waves has thrown much light on the constitution
of the interior of the earth.
* Jeffreys, H.: London, Man. Not. R. astr. Soc., geophys. Suppl., Vol. I, No 9.
t Turner, H. H.: London, Mon. Not. R. astr. Soc., geophys. Suppl., Vol. I, No.
J A. Sieberg : " Erdbebenkunde," Gustav Fischer, Jena, 1923.
(90565)
C*
EARTH TEMPERATURE
74
Earth Temperature.—The temperature of the interior of the earth is very
high, and in consequence heat flows by conduction from the interior towards
the cooler surface. If the temperature of the surface layer were constant
the flow of heat would be constant, and a steady gradient of temperature
would be established right up to the surface. In these circumstances, the
temperature differences at different depths would be approximately pro­
portional to the differences between the depths and temperature would always
increase as the depth increased.
This simple regime is profoundly affected by the fact that the temperature
of the surface layers of the earth is subject to large changes, some of which
are periodic and some irregular. By far the largest variations are increases
due to solar radiation and decreases due to terrestrial radiation, but conduction
of heat from the superincumbent air also plays an important part. The two
principal types of periodic variation are (1) seasonal and (2) diurnal.
If the surface layer is subjected to a steadily increasing temperature the
gradient of temperature in the first few inches of the soil is reversed, and
heat commences to flow downwards from the surface towards the layer of
minimum temperature. Below that layer the flow remains upwards from
the heated interior. The establishment of a flow of heat downwards takes
time but, on the other hand, when the flow is once established it will persist
after the increase of temperature at the surface has disappeared. If now
the temperature of the surface layer is steadily reduced the temperature
gradient in the first few inches becomes such that heat again flows upwards,
and we have a peculiar arrangement in which temperature downwards
increases at first, reaches a maximum, then diminishes, reaches a minimum
and finally increases again steadily towards the interior. Every variation
of temperature at the surface leads to a corresponding propagation of heat
upwards or downwards, but the rapid variations have not time to extend
very far down. The diurnal variation disappears at a depth of less than
3 ft. as a rule, but the seasonal variation has time to spread down much
further, to about 30 or 40 ft. At some such depth the temperature is sensibly
constant and below that depth there is a steady increase of temperature
downwards, the conditions being now unaffected by surface fluctuations of
temperature.
The following average values of temperature in the soil at Oxford illustrate
some of the above statements : at six inches, earth temperature is lowest
(38-5° F.) in December and highest (69-4° F.) in August; at ten feet, earth
temperature is lowest in April (47-5° F.) and highest in September (57-5° F.).
In April and also in September there is not much difference between earth
temperature at six inches and at ten feet, but at five feet earth temperature
in April is less than at six inches or ten feet, while in September it is greater.
In the summer months the temperature at six inches is much in excess of
that at ten feet, while in the winter months temperature steadily rises from
six inches to ten feet.
The variations of earth temperature produced by periodic variations of
the temperature of the surface layers have been studied mathematically, and
the results of the investigations have been verified by observation.
Earth Thermometer.—A thermometer for ascertaining the temperature of
the soil at a known depth. The commonest form (Symons's) consists of a
mercurial thermometer with its bulb embedded in paraffin wax, suspended
in a steel tube at depths of one foot or four feet. For depths of a few inches
only a mercurial thermometer with its stem bent at right angles, for
convenience in reading, is employed.
Eclipse Weather.—The influence which the eclipse exercises on solar
radiation received depends on a number of factors, but broadly speaking
the radiation at any time is proportional to the extent of the unobscured
sun. Light diminution is very gradual in the early partial phases, and very
75
ECLIPSE WEATHER
rapid just before totality. During the total phase light is received from two
sources, the solar appendages, chiefly the corona, and the partially illuminated
atmosphere lying at the moment just outside the umbral shadow. Of those
the former is by far the largest contributor, but the illumination from both
causes varies with the circumstances of individual eclipses.
The fall of temperature is one of the most clearly denned phenomena,
but is influenced by many factors. A table compiled by H. H. Clayton of
12 total eclipses from 1878 to 1905, after correction for diurnal variation,
shows maximum falls varying from 1-5°F. to 8-1° F. The amount of
fall varies with the character of the station, being for the eclipses cited
1-5° F. over the open ocean, 3-2° F. on a small tropical island, and 5-5° to
8-1° F. over large land areas. Over land areas the fall shows a latitude effect
with a minimum in high latitudes and a maximum at the equator. The
recovery of normal values appears to take place simultaneously with the end
of the eclipse, in the upper air, over the ocean and on small islands, but is
delayed from 1 to 2£ hours after the end of the eclipse in continental situations.
The temperature normally begins to fall 20 minutes after first contact, and
the minimum occurs from 2 to 20 minutes after totality. Kite meteorograph
observations from the " Otaria " in 1905 gave a fall of only 1° F. at 300m.
above the Atlantic Ocean. Temperature variation is, therefore, probably
confined to a very shallow layer not exceeding 300m. or 400m. above the
earth's surface. At hill stations the fall is usually less than at low-level
stations, but comparisons are not always consistent.
Changes of vapour pressure are more difficult to distinguish, but there
appears to be an increase of vapour pressure up to a time 30 to 50 minutes,
preceding totality, a minimum pressure at totality, and a second maximum
about 30 minutes afterwards. This is characteristic only of stations in or
near the total belt, but applies both on the ground and at 300m. The decrease
is as large over the ocean as over large land areas. Relative humidity is
acted on by opposing factors, the decrease in vapour pressure and the decrease
in temperature. Over land areas the relative humidity usually rises to a
maximum at the time of minimum temperature through preponderance of
the second factor. Dew is often formed on the ground near the time of
mid-eclipse. Cloud changes which appear to be definitely associated with
the eclipse and also the formation of fog have been noted.
The variation of atmospheric pressure is not conspicuous with ordinary
instruments, but observations with these and with microbarographs have
produced considerable but not conclusive evidence to show that the eclipse
curve has two minima and three chief maxima, the latter occurring at totality
and about 75 minutes before and after it. The fluctuation is of the order
of 0-2 mb.
A diminution in wind velocity is a feature almost as marked as the tem­
perature fall. The minimum usually agrees closely with the time of minimum
temperature. There are also strong evidences of maxima represented by
gusts of increased velocity occurring 30 to 50 minutes before and 40 to
60 minutes after totality.
The production of winds with definite cyclonic circulation has been
observed on some occasions but not on others. The resultant eclipse wind,
however, frequently shows a definite succession of changes, with a reversal
of direction before and after totality; it can never be large as the
temperature gradient produced by the shadow is comparatively small.
The optical phenomena are of considerable interest; the colours of sky,
clouds and landscape are usually very striking and subject to rapid changes
about the time of totality. While the sun is completely obscured the light
is of very peculiar quality, and appears to justify adjectives such as " livid "
and " unnatural/' by which it is usually described. Vitiation of distance
estimates is a noticeable feature at this time. The approaching umbral
shadow is indigo or blackish, and may be seen, from an elevated position,
(90565)
C*2
EDDY
76
to pass points at known distances. As it reaches the observer definite pulsa­
tions, probably diffraction phenomena, may be observed. The best known
of eclipse optical phenomena are the " shadow-bands." These are observed
about 5 minutes before totality, and therefore definitely before the umbral
shadow has reached the observer, and again after it has left him. The shadowbands thus have no connexion with the pulsation of the shadow edge. The
phenomenon consists of undulating dark bands, separated by light spaces,
following one another in a direction usually normal to the tangent to the
crests. The bands are visible on the ground, but are best seen if a sheet is
laid down, or on snow, or on the walls of white houses. The bands appear to
flicker and, while sometimes of considerable regularity, are often very vague
and confused. Their speed is also inconstant, varying from about 3 to
25 m.p.h., and, therefore, very much slower than the speed of the moon's
shadow. This is one of several facts which have disproved the theory that
the bands are diffraction fringes bordering the moon's shadow. The bands
were photographed for the first time in 1925 in the United States, and they
have been observed up to a height of 3,800m. on sheets hung below manned
balloons. The general inference is that they are caused by the diminishing
crescent of light penetrating air strata differing in their thermal and hygrometrical conditions and, therefore, in their refractive power.
Eddy.—A name given to the deviation from steady motion which occurs
in any viscous fluid which flows past any obstacle, or in which neighbouring
streams flow past or over each other. Certain types of eddies, such as those
formed in water at the side of a moving vessel, and those which sometimes
form at street corners, are of the nature of whirls, but in meteorology the
restriction of the use of the name eddy to whirling motions is not justifiable.
See Turbulence.
Eddy Viscosity.—According to the kinetic theory of gases, the viscosity
of a fluid is due to the motion of molecules in a field where a velocity gradient
exists, resulting in the transfer of momentum from one layer of the fluid
to another. For the simplest molecular model, the coefficient of viscosity is
given by (i = $pcl, where p is the fluid density, c is the average molecular
velocity and / is the length of the free path. This concept of the transfer of
momentum by secondary motion perpendicular to the mean flow has been
applied to explain phenomena in turbulent medium, taking the eddies to be
carriers of momentum (Prandtl) or vorticity (Taylor). The quantity
corresponding to the coefficient of viscosity is now A = pwl, where w is the
eddy velocity in the direction of transfer (usually the vertical), and / is a
length termed the " mixing path " (Mischungsweg), and somewhat loosely
defined as the distance traversed by an eddy before mixing with the
surrounding fluid. With this definition the eddy shearing stress is
r = A du/zd, where u is the mean velocity, and z is the transverse co-ordinate.
Unlike the molecular coefficient, the quantity A, which in continental
literature is termed the " interchange coefficient " (Austauschkoeffizient) is,
in general, a function of position. In English meteorological writings, the
quantity K = A/p, is used, and is generally termed the " eddy viscosity."
In 1915, G. I. Taylor, by considering two dimensional motion and making
use of the conservation of vorticity of a moving element, was able to
apply these ideas with considerable success to the atmosphere. Taking K
independent of height, the approach of the actual wind to the geostrophic wind
can be calculated and compared with pilot balloon observations. The results,
on the whole, reproduce reasonably well the main features of wind structure
from about 100 ft. to 2,000 ft. Taylor found that K was of the order of
104 cm. 2/sec., a value in reasonable agreement with that found for the eddy
conductivity of heat. The scale of eddy transfer of momentum is thus vastly
greater than that of molecular transfer.
For heights below 100 ft. or so, the assumption that the eddy viscosity
is independent of height leads to expressions for the wind profiles which are
ELECTRICAL UNITS
77
not in agreement with observations. It is found that the wind structure in
these regions may be represented as a power of the height, or better, as a
linear function of the logarithm of the height. The index in the power law
and the parameters in the logarithmic law are functions of the roughness
of the surface and of the degree of atmospheric stability. It thus follows
that, near the ground, the eddy viscosity itself varies considerably with
height, the nature of the surface and the degree of atmospheric stability.
Effective Height of an Anemometer.—The height at which a given
anemometer would register a mean velocity equal to that actually observed,
if there were no obstructions such as buildings or trees in its vicinity. When
there are obstructions around the site of an anemometer it is the practice to
raise its vane to such a height above ground that the effective height is equal
to the standard height in an unobstructed situation, namely 10 metres or
33 ft. In these circumstances the recorded mean velocity can be converted
to wind force on the Beaufort Scale by means of the ordinary table of velocity
equivalents. (See " Meteorological Observers Handbook ".)
Electrical Units.—The definitions of the absolute electrical and magnetic
units will be found in treatises on electricity. We note here that the
fundamental relation for the electrostatic group is the definition of a unit
of charge, viz., the repulsion between two unit charges of electricity at unit
distance is unit force, whilst the fundamental relation for the electromagnetic
group is the definition of the unit magnetic pole, viz., the repulsion between
two unit magnetic poles at unit distance is unit force.
The number of electrostatic units in an electromagnetic unit is vn where v
is the number which represents the velocity of light (which number is
3 X 10 10 , or more precisely 2-997 X 10 10, in the C.G.S. system) and the
index n is 0, i 1 or i 2.
The practical electrical units are defined by their relations to the
corresponding C.G.S. electromagnetic units. The relations to the electrostatic
units are also shown in the following list.
n
Quantity of Electricity
Electric current
Potential
Resistance
Induction
Energy
Power
Capacity
1
1
-1
_2
-2
0
0
2
Practical unit
Coulomb
Ampere
Volt
Ohm
Henry
Joule
Watt
Farad
}
}
Electromagnetic
C.G.S.
Electrostatic
C.G.S.
1010"
3 x 10*
1/300
10
imi\)
1/(9X10
10'
10'
io-»
9 X 10"
The C.G.S. unit of magnetic force is the Oersted. It is such that unit
magnetic pole in a field of 1 oersted experiences a force of 1 dyne and is
connected with the electrical units by the relation that it is the force pro­
duced at a distance of 1 cm. from an infinitely long straight wire carrying a
current of 5 amperes. The C.G.S. unit of magnetic induction is the Gauss.
The practical unit in geomagnetism is the Gamma, which is 10~ 5 C.G.S.
units. In free air the numerical values of magnetic force and magnetic
induction are the same ; thus at Eskdalemuir in 1936
the average total magnetic force
= 47849y
= 0-47849 oersted
„
,,
,,
„
induction = 0-47849 gauss
In atmospheric electricity the relation between potential gradient and
induced charge is frequently required. This relation is
F=4n£a
ELECTRICITY
78
where a the density of the charge is measured in coulombs per square centimetre,
F the strength of the field is measured in volts per centimetre, and f is equal
to 9 x 10".
A convenient symbolism for the C.G.S. units, has been introduced by
Prof. H. Benndorf. Noting that c is the initial letter of coulomb, the practical
unit of electric charge, he writes cem and ces for the units of charge in the
electromagnetic and electrostatic systems. Similarly aem and aes are units
of current and so on. The indivisible quantity of electricity, the electronic
charge, is equal to 4-8 x 10~ 10 ces and therefore to 1-59 x 10~ 19 coulomb.
Electricity.—See ATMOSPHERIC ELECTRICITY.
Electricity of Precipitation.—Precipitation, both solid and liquid, is nearly
always electrically charged when it reaches the ground. The charge may be
positive or negative and drops with charges of either sign may fall
simultaneously. The average charge per cubic centimetre of rain is of the
order of one electrostatic unit (1/3 X 10~9 coulomb) but charges of about
20 e.s.u. per cm.3 have been observed and, in the case of individual drops,
the intensity of electrification may be as high as 200 e.s.u. per cm.3 Snow is
usually more highly charged than rain. Since the charges vary over such a
wide range it is necessary to carry out a long series of observations in order
to ascertain the net charge carried down by precipitation at any particuar
place. It is well established that much more rain is charged positively than
negatively (the ratio being roughly 3:1), but the excess of positive charge
carried to earth is not proportionately large since the intensity of
electrification is generally considerably greater when the precipitation is
negatively charged than when it is positively charged. Of the types of rain
that are prevalent in western Europe it is the showery type that is responsible
for most of the high negative charges whilst thunderstorm rain is usually
associated with excess of positive charge, the heavier rain at the commence­
ment of a storm being positively charged and the more moderate rain of
later stages tending to be negatively charged ; ordinary continuous rain
nearly always carries a small positive charge*. During positively charged
rain the potential gradient at the ground is much more often negative than
positive, but during negatively charged rain there is no marked tendency
for the gradient to be of one sign rather than the other although there may be
violent fluctuations in sign.
The results of measurements of the electricity of rain throw considerable
light on the causes of the electrical phenomena associated with disturbed
weather conditions ; on the other hand they add to the difficulty of explaining
the phenomena in terms of a simple theory. It appears that there must be at
least two distinct physical processes by which the electrification of precipitation
is produced.f In the upper parts of clouds where the temperature is usually
well below freezing point it is probable that the impact of ice crystals is the
main process by which a separation of electricity takes place, the ice
becoming negatively charged and the air positively charged. Precipitation
originating as ice crystals will tend to carry down negative charge leaving
the upper part of the cloud positively charged, but it does not necessarily
follow that all precipitation originating in this manner will be negatively
charged since the smaller ice crystals which sink less rapidly will tend to collect
the positive charge accumulating at the top of the cloud. A second process
of separation of electricity comes into play in well-developed thunder­
storms in which the strong vertical air currents are able to break up large
water drops into smaller drops, the water becoming positively charged and
air negatively charged.J Localised regions of positive charge thus develop,
generally at the base near the front of a thundercloud, and the heavy rain
which falls from such a region nearly always carries a high positive charge.
* Scrase, F. J.; London, Geophys. Mem., No. 75, 1938.
t Simpson, G. C. and Scrase, F. J. ; London, Proc. roy. Soc., A. 161, 1937, pp. 309-52.
j Simpson, G. C.; London, Proc. roy. Soc., A. 114, 1927, pp. 376-401.
79
ENERGY
There is a third process of electrification which may become effective in the
lowest layers of a cloud or between cloud and ground where the precipitation
is in the liquid stage; this is the influence process* by which water drops
falling sufficiently fast in an electric field, tend, on account of the induced
charges on their upper and lower halves, to attract ions of opposite sign to
that of the field. This mechanism suggests that the electrification of rain is
the result rather than the cause of disturbed potential gradient, whereas in
actual fact it is not until a cloud turns to rain that the potential gradient is
appreciably disturbed.
The net transfer of positive electricity from the atmosphere to the earth
by precipitation constitutes a current which is on the whole in the same
direction as the fine weather conduction current and this fact only adds,
therefore, to the difficulty in explaining how the earth's electric charge is
maintained.
Electrometer.—An instrument for measuring electromotive force or
potential difference. Its function is, therefore, similar to that of a voltmeter,
but electrometers are distinguished by the fact that their action depends on
electrostatic force, whereas ordinary voltmeters depend for their action on the
magnetic or heating effects of an electric current. The indications of an
electrometer do not involve the passage of a current through the instrument,
and they can, therefore, be used for such purposes as the determination of the
electrical potential of a point in the atmosphere.
The instrument generally used for absolute measurements of potential
gradient is the Wulf bifilar electrometer. For continuous records a quadrant
electrometer is generally used, the registration being photographic.
Autographic records can also be obtained with the Benndorf quadrant electro­
meter, which is provided with a guillotine to depress the " needle " at regular
intervals and so produce marks on paper. For recording photographically
rapid changes of potential, a Dolezalek quadrant electrometer can be used with
the period reduced to about a second. Measurements of potentials such as a
fraction of a volt can conveniently be made with a Lindemann electrometer,
as for instance in observations of conductivity.
Electroscope.—An instrument for indicating the presence of electrification,
usually by some simple action, such as the mutual repulsion of two strips
of foil.
Element.—In meteorology one of the components which determines the
state of the atmosphere at any given time and place. The chief meteorological
elements are temperature, pressure, wind, precipitation, humidity, cloudiness.
Atmospheric electricity and atmospheric pollution are sometimes included.
Energy.—Used frequently in meteorology in the general sense of vigour
or activity. Thus, a cyclone is said to develop greater energy when its
character, as exhibited by a low barometer, steep gradients and strong winds,
becomes more pronounced. But there is a technical dynamical sense of the
word, the use of which is sometimes required in meteorology, and which
must become more general when the physical explanation of the phenomena
of weather is studied, because all the phenomena of weather are examples
of the " transformations of energy " in the physical sense.
The most important conception with regard to energy is its division into
two kinds, kinetic energy and potential energy, which are mutually convertible.
A clock weight gives a good idea of potential energy. When the clock is wound
up the weight has potential energy in virtue of its position ; it will utilise
that energy in driving the clock until it is " run down " and can go no further.
Potential energy must be restored to it by winding up before it can do any
more driving. The potential energy in this case is measured by the amount
* Wilson, C. T. R.; Philadelphia, J. Franklin Inst,, 208, 1929, pp. 1-12.
ENTROPY
80
of the weight and the vertical distance through which it is wound up. In
dynamical measure the potential energy of the raised weight is mgh, where m
is the mass of the clock weight, h the vertical distance through which it is
wound up, g the acceleration of gravity. It is to the mysterious action of
gravity that the energy is due :—hence the necessity for taking gravity into
account in measuring the energy.
Using the simple product mgh as a measure of the potential energy of
gravitation, by a simple formula for bodies falling freely under the action
of gravity, we have
mgh = tynv*
where v is the velocity acquired by a body falling through a height h, or,
speaking in terms of energy, by losing the potential energy of the height h.
It thus obtains a certain amount of motion which represents kinetic energy,
in exchange for its potential energy. The kinetic energy is expressed by
the apparently artificial formula \mv z . In virtue of its motion it has the
power of doing " work " : if it were not for unavoidable friction the mass
could get itself uphill again through the height A by the use of its motion,
and thus sacrifice its kinetic energy in favour of an equivalent amount of
potential energy.
The exchange of potential and kinetic energy can be seen going on in a
high degree of perfection in a swinging pendulum. At the top of the swing
the energy is all potential, at the bottom all kinetic. The swings get gradually
smaller because in every swing a little of the energy is wasted in bending the
cord or in overcoming the resistance of the air.
What we get in return for the loss of energy in friction is a little HEAT,
and one of the great conclusions of physical science in the middle of the
nineteenth century was to show that heat is also a form of energy, but a
very special form, that is to say, its transformation is subject to peculiar
laws. Heat is often measured by rise of temperature of water (or its equivalent
in some other substance). Calling this form of energy thermal energy and
measuring it by the product of the " water equivalent," M, and the rise of
temperature 6 — 6 0 produced therein, we have three forms of energy all
convertible under certain laws, viz. :—
Potential energy
..
..
mgh.
Kinetic energy
..
..
\vnvz.
Thermal energy
..
..
M(B — 0 0).
We have mentioned only a lifted clock weight as an example of potential
energy, but there are many others, a coiled spring that will fly back when
it is let go, compressed gas in a cylinder that will drive an engine when it is
turned on, every combination, in fact, that is dormant until it is set going
and then becomes active.
From the dynamical point of view, the study of nature is simply the
study of transformations of energy.
In meteorology kinetic energy is represented by the winds ; potential
energy by the distribution of pressure at any level, by the electrical potential
of the air and by the varying distribution of density in the atmosphere,
causing convection ; thermal energy by the changes of temperature due to
the effect of the sun or other causes. It is the study of the interchange of
these forms of energy which constitutes the science of dynamical meteorology.
Entropy.—A term introduced by R. Clausius to be used with TEMPERATURE
to identify the thermal condition of a substance with regard to a
transformation of its HEAT into some other form of ENERGY. It involves
one of the most difficult conceptions in the theory of heat, about which some
confusion has arisen.
The transformation of heat into other forms of energy, in other words
the use of heat to do work, is necessarily connected with the expansion of
the working substance under its own pressure, as in the cylinder of a gas
engine, and the condition of a given quantity of the substance at any stage
81
ENTROPY
of its operations is completely specified by its volume and its pressure.
Generally speaking (for example, in the atmosphere), changes of volume and
pressure go on simultaneously, but for simplifying ideas and leading on to
calculation it is useful to suppose the stages to be kept separate, so that
when the substance is expanding the pressure is maintained constant by
supplying, in fact, the necessary quantity of heat to keep it so, and, on the
other hand, when the pressure is being varied the volume is kept constant;
this again by the addition or subtraction of a suitable quantity of heat. While
the change of pressure is in progress, and generally also while the change of
volume is going on, the temperature is changing, and heat is passing into or
out of the substance. The question arises whether the condition of the
substance cannot be specified by the amount of heat that it has in store and
the temperature that has been acquired just as completely as by the
pressure and volume.
To realise that idea it is necessary to regard the processes of supplying
or removing heat and changing the temperature as separate and independent,
and it is this step that makes the conception useful and at the same time
difficult.
For we are accustomed to associate the warming of a substance, i.e. the
raising of its temperature, with supplying it with heat. If we wish to warm
anything we put it near a fire and let it get warmer by taking in heat, but
in thermodynamics we separate the change of temperature from the supply
of heat altogether by supposing the substance is " working." Thus, when
heat is supplied the temperature must not rise ; the substance must do a
suitable amount of work instead ; and if heat is to be removed the temperature
must be kept up by working upon the substance. The temperature can
thus be kept constant while heat is supplied or removed. And, on the
other hand, if the temperature is to be changed it must be changed dynamically
not thermally, that is to say, by work done or received, not by heat
communicated or removed.
So we get two aspects of the process of the transformation of heat intoanother form of energy by working, first, alterations of pressure and volume,
each independently, the adjustments being made by adding or removing heat
as may be required, and secondly, alterations of heat and temperature inde­
pendently, the adjustments being made by work done or received. Both
represent the process of using heat to perform mechanical work or vice versa.
We could, therefore, specify the condition of a given quantity by H
and T, its store of heat and its temperature. Further consideration wil>
show, however, that the quantity H is not very suitable for the purpose.
When a substance expands adiabatically it is doing work, and since it is
receiving no heat from an outside source it must draw upon its own effective
internal supply of heat. Hence the latter diminishes in direct ratio to the
decrease of absolute temperature and the value which remains constant is
not H but the ratio H/T. We call this ratio the entropy, an adiabatic line
which conditions thermal isolation and therefore equality of entropy is
called an isentropic. If a new quantity of heat h is added at a temperature T
the entropy is increased by h/T. If it is taken away again at a lower
temperature T' the entropy is reduced by h/T'.
In the technical language of thermodynamics the mechanical work for
an elementary cycle of changes is Sp. 8v, and the element of heat ST. 8<j>.
The conversion of heat into some other form of energy by working is expressed
by the equation
ST. 8<j> - Sp. Sv
when heat is measured in dynamical units.
Entropy is a relative term ; it is also an abstraction and cannot be
measured directly. Its importance lies in the assistance which it gives in
deriving thermodynamic relationships. Problems of dynamical meteorology
seem to be more closely associated with these strange ideas than those which-
EQUATION OF TIME
82
we regard as common. For example, it may seem natural to suppose that if
we could succeed in completely churning the atmosphere up to, say,
10 kilometres (6 miles) we should have got it uniform in temperature or
isothermal throughout. That seems reasonable, because if we want to get a
bath of liquid uniform in temperature throughout we stir it up ; but it is
not true. In the case of the atmosphere there is the difference in pressure
to deal with, and, in consequence of that, complete mixing up would result
not in equality, but in a difference of temperature of about 100° C. between
top and bottom, supposing the whole atmosphere dry. The resulting state
would not, in fact, be isothermal; the temperature at any point would
depend upon its level and there would be a temperature difference of 1° C. for
every hundred metres. But it would be perfectly isentropic. The entropy
would be the same everywhere throughout the whole mass. And its state
would be very peculiar, for if you increased the entropy of any part of it
by warming it slightly the warmed portion would go tight to the top of the
isentropic mass. It would find itself a little warmer, and, therefore, a little
lighter specifically than its environment, all the way up. In this respect we
may contrast the properties of an isentropic and an isothermal atmosphere.
In an isentropic atmosphere each unit mass has the same entropy at all
levels, but the temperatures are lower in the upper levels. In an isothermal
atmosphere the temperature is the same at all levels, but the entropy is greater
at the higher levels.
An isothermal atmosphere represents great stability as regards vertical
movements, any portion which is carried upward mechanically becomes
colder than its surroundings and must sink again to its own place, but an
isentropic atmosphere is in the curious state of neutral equilibrium which
is called " labile." So long as it is not warmed or cooled it is immaterial
to a particular specimen where it finds itself, but if it is warmed, ever so
little, it must go to the top, or if cooled, ever so little, to the bottom.
In the actual atmosphere above the level of ten kilometres (more or less)
the state is isothermal; below that level, in consequence of convection, it
tends towards the isentropic state, but stops short of reaching it by a variable
amount in different levels. The condition is completely defined at any level
by the statement of its entropy and its temperature, together with its
composition which depends on the amount of water vapour contained in it.
Speaking in general terms the entropy increases, but only slightly, as
we go upward from the surface through the TROPOSPHERE until the
STRATOSPHERE is reached, and from the boundary upwards the entropy
increases rapidly.
If the atmosphere were free from the complications arising from the
condensation of water vapour the definition of the state of a sample of air
at any time by its temperature and entropy would be comparatively simple.
High entropy and high level go together; stability depends upon the air
with the largest stock of entropy having found its level. In so far as the
atmosphere approaches the isentropic state, results due to CONVECTION may
be expected, but in so far as it approaches the isothermal state, and stability
supervenes, convection becomes unlikely.
Equation of Time.—See TIME.
Equatorial Air.—See TROPICAL AIR.
Equiangular Spiral.—A spiral is the locus of the extremity of a line (or
radius vector) which varies in length as it revolves about a fixed point. The
equiangular spiral is such that as the vector angle increases arithmetically
the radius vector increases geometrically. Another definition is that the
tangent makes a constant angle a with the radius vector. The equation in
polar co-ordinates is r — ae^co^a'
The study of turbulence led Hesselberg and Sverdrup to a theoretical result
involving this type of spiral, namely that if the wind at successive levels above
the ground is represented by a series of vectors drawn from the point at which
83
ERROR
the wind is measured, the extremities of the vectors lie on an equiangular
spiral with its pole at the end of the vector representing the geostrophic wind.
See WIND.
FIG. 18.
Equinox.—The time of the year when the astronomical day and night are
equal, each lasting twelve hours. At the equinox the sun is " on the equator "
or is " crossing the line." It is on the horizon in the morning exactly in the
east, and exactly in the west in the evening all over the world. Sunrise occurs
at the same time all along a meridian. The sun is visible by refraction for
a little longer than the duration of the astronomical day.
There are two equinoxes. The spring or vernal equinox about March '21,
and the autumnal equinox about September 22.
Equivalent Constant Wind.—A particular type of average wind of great
importance in BALLISTICS. It may be defined as that wind, constant in
speed and direction, which would produce the same displacement of a shell
in flight as the actual varying winds which the shell meets during its flight.
Equivalent Potential Temperature.—See ROSSBY DIAGRAM.
Equivalent Temperature.—Equivalent temperature may be denned as the
temperature of absolutely dry air which has the same wet-bulb temperature
as the actual air.
Error.—In any observation other than the counting of discrete units,
such as the number of balls in a basket, the repetition of the observation does
not usually yield precisely the same result. This is particularly the case
when an instrument is used to yield a measurement to the greatest possible
number of significant figures. Successive measurements will then frequently
give varying values for the last significant figure. It follows that no single
observation can be regarded as strictly correct. The difference between the
observed value and the true value is called the " error " of the observation.
In the discussion of errors of observation it is usually assumed that the
observations have been carried out with the greatest possible care. Just
as an observer may make a mistake in counting the number of balls in a
basket, so he may commit a mistake of 5° in reading a thermometer. Such
a mistake should be. classified as a " blunder " and not as an " error." But
even when all possible care has been taken to avoid blunders there still
remain errors which cause individual observations in a series to differ from
one another and from the true value of the quantity to be measured. These
residual errors are due to the peculiarities of the observer, of the instrument,
or of the external conditions.
EVAPORIMETER
84
The observer may have individual peculiarities, particularly in measuring
positions of moving bodies, or estimating the instantaneous magnitude of a
changing length. A good observer tends always to make the same error in
observing, whereas an inexperienced observer is more erratic, both as to the
magnitude and sign of his error. The personal error or " personal equation "
of an observer can in some cases be estimated by comparison with a standard
observer, or by means of specially designed laboratory experiments.
The instrument may have errors of construction or of graduation. These
can generally be eliminated by comparison with a standard instrument.
They may be constant errors, always of the same sign and magnitude, as in
the case of a barometer whose scale has been displaced vertically from its
correct position ; or they may be systematic, bearing a fixed relation to one
or more of the conditions of observation. For example, if a thermometer
be constructed by graduating a degree scale upon a glass tube at equal intervals,
any inequalities in the bore of the tube will lead to systematic errors which
can be eliminated by comparison with a standard instrument.
Changing external conditions may also lead to errors in observation.
By external conditions we mean such of the conditions of observation as are
beyond the observer's control, as for example the vibration of a theodolite
by the wind. It is seldom possible to make a correction for such conditions.
All that can be done is to select such times of observation as shall be as little
as possible subject to uncontrollable external conditions, when such selection
is possible.
To obtain accurate observations with an instrument we therefore require
to know its constant or systematic error. If the observer has a personal
error, an effort should be made to estimate it, and allow for it. When these
constant or systematic errors have all been allowed for as far as possible, it will
generally be found that the individual observations in a series still differ from
one another. The residual error which causes this difference is called the
" accidental " error of the observation. The word " accidental " is here used
in a special sense, to denote the sum total of all the errors whose source is
unknown. They may be due for example to changes in the observer, as when
an observer, becoming tired, makes errors of varying magnitude and sign in
different observations; or to unsuspected temperature changes in the
instrument; or to irregular effects of wind upon the instrument or upon the
object observed. They may also be due partly to failure to realise the nature
of some of the factors affecting the quantity measured, or even to defects in
the theory on which the reduction of the observations is based. The endeavour
of the observer should be to take account of all changing conditions as far as
possible, and so to remove as much of the errors of observation as he can into
the class of constant or systematic error.
Some judgment is, however, necessary in the application of these
principles in meteorology. In climatological observations, for example, it is a
waste of time to measure the air temperature to -01° C., since the fluctuating
changes of temperature at a given place from one minute to the next generally
exceed this value. Reading to • 1° C. is, however, readily possible.
See also PROBABILITY, PROBABLE ERROR, NORMAL LAW.
Evaporimeter.—An instrument for determining the rate of evaporation
of water into the atmosphere. Evaporimeters fall into two classes, (a) those
employing a free water surface, (b) those in which evaporation takes place
from a continuously wetted porous surface of blotting paper, fabric or ceramic
material. Class (a) is exemplified by the Symons evaporation tank commonly
used in this country. It consists of a galvanized iron tank usually 6 ft.
square and 2 ft. deep, nearly filled with water and sunk into the ground
with its rim projecting about \\ in. The evaporation, expressed as depth
in inches, is deduced from daily readings of level made with a micrometer
gauge, allowance being made for rainfall. Evaporimeters of class (b) have
been designed by Pich6, Livingston, Owens and others but they have not
come into general use in this country as meteorological instruments.
Expansion.—The increase in the size of a sample of material, which
may be due to heat or to the release of mechanical strain, or the absorption
of moisture or some other physical or chemical change.
EXPOSURE
85
The size may be taken as the length or volume, sometimes as the area.
In the science of heat the fractional increase of length or volume for one
degree of temperature is called the coefficient of thermal expansion. Thus
the coefficient of " linear " expansion with heat of the brass used for barometer
scales is 0-0000102 per degree Fahrenheit, which means that for 1° F. the
length of the scale increases by 102 ten-millionth parts of its length at the
standard temperature (62° F.). The coefficient of " cubical " expansion of
mercury is -0001010, which means that the volume of a quantity of mercury
increases by 1-01 ten-thousandth of its bulk at the standard temperature
(32° F.) for 1°F. The corresponding expansions and coefficients for 1°C.
or 1° A. are larger in the ratio of 18 to 10.
Expansion of volume alters the density of a substance, and changes of
density are therefore numerically related to expansion.
The expansion of a gas may be caused either by reduction of pressure
or by increase of temperature. So in order to see the effect of temperature
alone we must keep the pressure constant. In these circumstances the
coefficient of expansion is -00366 for 1°A. referred to 273° A. as standard,
or -002 for 1° F. referred to 41° F. as standard.
Evaporation.—The process of conversion of water or ice into aqueous
vapour. The atmosphere is very rarely completely saturated, so that
evaporation is nearly always going on, and in general the greater the difference
between the amount of water vapour which the air holds and the amount
which would saturate it at the existing temperature, the more rapid is the
rate, but other factors such as the nature of the water surface from which
evaporation is taking place, and wind, influence the rate.
Some figures for the average depth of water evaporated in various places
are given below.
Evaporation from the Surface of Water
35-year
average.
London,
Camden
Square.
1937
Great
Britain,
mean of 14
stations.
South
Africa,
Bulawayo.
20° 2' S.,
28° 58' E.
Egypt,
Wadi
Haifa.
21°55'N.,
31°20'E.
Tasmania,
Hobart.
42°53'S.
27° 27' S.,
117° 52' E. 147°20'E
Australia,
Cue, W.A.
January
February . .
March
in. mm. in. mm.
•10 2-5 •20 5-1
•25 6-4 •22 5-6
•66 16-8 •47 11-9
in.
mm. in.
mm. in.
mm.
8-94 227 11-10 282 21-46 545
7-40 188 12-64 321 17-13 435
8-74 222 17-96 456 16-78 426
April
May
June
1-51 38-4 1-21 30-7
2-43 61-7 2-44 62-0
2-91 73-9 3.00 76-2
8-90
9-02
7-99
226 21-85
229 25-52
203 25-28
555 11-22
648 7-48
642 4-84
285 1-97
190 1-22
123 0-67
50
31
17
July
August
September . .
2-97 75-4 2-65 67-3 8-07
2-33 59-2 2-52 64-0 12-84
1-38 35-1 1-64 41-7 14-73
205 23-82
326 22-84
374 22-92
605
580
582
5-35
6-26
9-13
136 0-83
159 1-30
232 1-77
21
33
45
•81 20-6 14-96
•34 8-6 10-79
•27 6-9 9-84
380 20-08
274 15-12
250 10-75
510 13-39
384 18-00
273 22-37
October
November . .
December . .
Year
•62 15-7
•25 6-4
•09 2-3
in. mm
5-87 149
4-21 107
3-03 77
340 3-07 78
457 4-02 102
568 4-88 124
15-50 393-8 15-77 400-6 122-22 3,104 229-88 5,838 153-41 3,896 32-84 834
For any area where the percolation is small the amount of evaporation is
given by the difference between the rainfall and run-off. Statistics of run-off
and rainfall for a number of areas are published by the Ministry of Health
in the annual volumes of the " Surface Water Year Book."
Exposure.—In meteorology, the method of presentation of an instrument
to that element which it is destined to measure or record, or the situation of
the station with regard to the phenomenon or phenomena there to be observed.
If meteorological observations are to be of full value attention must be paid
to the manner of the exposure of the instruments. Details are to be found in
the " Meteorological Observer's Handbook." Uniformity of exposure is of
the greatest importance, and for that reason the pattern of the thermometer
screen has been standardised in most countries, while in these Islands a
standard height of one foot above ground for the rain-gauge has been fixed.
EXSICCATION
86
It is important, too, that the sites of the thermometer screen and rain-gauge
should not be unduly shut in ; on the other hand, a very open exposure, as
on a bare moor, is undesirable for a rain-gauge, as is also a position on a slope,
a roof, or near a steep bank. In these cases the catch is reduced by the effect
of wind eddies due to the obstruction of the gauge itself. A SUNSHINE
RECORDER demands an entirely unobstructed horizon near sunrise and sunset
at all seasons of the year. The question of the exposure of ANEMOMETERS is
one of great difficulty. The effect of the ground on a uniform current of air
blowing above it is to reduce the velocity, the amount of reduction increasing as
the ground is approached, and, at the same time, to introduce unsteadiness
into the motion which is manifested by the creation of eddies in the air.
The motion is then said to be turbulent (see TURBULENCE). A recording
tube anemometer erected in a turbulent wind shows a large number of gusts
and lulls corresponding with various parts of the eddy-motion. Hence the
extent of the gustiness of the wind as recorded by the anemometer (see GUST)
is a fair index of the excellence of the exposure. The ideal exposure for an
anemometer is at the top of a pole 30 or 40 ft. high, erected on a flat treeless
plain. Trees and buildings introduce much turbulence into air motion, and
where these occur in the vicinity of an anemometer the head of the instrument
is raised with a view to making the EFFECTIVE HEIGHT equivalent to the
standard value (10 m. or 33 ft.).
Exsiccation.—Drying by the draining away or driving away of moisture.
The term implies some change, frequently the result of human agency
which decreases the quantity of moisture available without any appreciable
change in the average rainfall. It is used in contrast with DESICCATION,
which implies an actual drying up due to a change of climate. Examples
of exsiccation are the washing away of the soil due to the cutting down of
forests, with the consequent conversion of a fertile region into the semblance
of a desert, the advance of sand dunes across cultivated ground, and the
draining of swampy ground.
Extremes.—The highest and lowest values of meteorological elements.
Standard climatological tables give the highest and lowest temperatures
recorded at an observing station in the day, month and year. The mean
daily maximum temperature is the average of the maxima for each day,
similarly for the mean daily minimum. The monthly maximum and
minimum temperatures are the highest and lowest temperatures recorded
during the month. The highest and lowest temperatures during the whole
period of the observations are called the absolute extremes.
The absolute extremes of temperature are :—For the British Isles :
highest, 100-5° F. at Tonbridge, July 22, 1868; lowest, — 17° F. at
Braemar, February 11, 1895.
For the surface of the globe: highest,
136° F. at Azizia (Uzzizia), Tripoli, September 13, 1922 ; lowest, — 93-6°F.
at Verkhoiansk, Siberia, January 3, 1885. In the upper air: lowest,
—133-4°F. at a height of 16£ Km. over Agra, India on October 4, 1928.
The highest known pressures corrected to M.S.L. are : British Isles,
1055 mb., in Scotland on January 9, 1896 ; World, about 1079 mb., in Siberia,
January 23, 1900. The lowest pressures are : British Isles, 926 mb., in
Scotland on January 31, 1902 ; World, 887 mb., in the Pacific Ocean on
August 18, 1927.
The extreme annual or seasonal totals of rainfall are important in
agricultural countries, and maximum values of rainfall in a short period are
of interest to engineers. The extreme annual total in the British Isles is
247 in., at Llyn Llydaw, Snowdon, in 1909.
The heaviest known rainfalls in 24 consecutive hours are : British Isles,
9-56 in., at Bruton, Somerset on June 28, 1917 ; World, 45-99 in., in the
Philippine Isles on July 14-5, 1911.
The maximum wind velocities in gusts, as shown by pressure-tube
anemographs, are also important to engineers. The extreme GUST recorded
for the British Isles is 113 mi./hr., at St. Ann's Head on January 18, 1945.
The highest hourly wind velocity is 78 mi./hr. (34-9 m./sec.), at Fleetwood
on December 22, 1894, recorded by a Robinson cup anemometer.
87
FLOE
Eye of Storm.—The central calm area of a tropical cyclone. The most
noticeable feature of this area is the sudden drop in wind from hurricane
force to light unsteady breezes or even to a complete calm, with more or
less cloudless sky and absence of rain. The interior margin of the storm
appears to be symmetrical around the calm centre. Over the ocean the sea
in the eye of the storm is usually very high and turbulent.
Eye of Wind.—A nautical expression indicating the direction from which
the wind blows.
Fahrenheit, Gabriel Daniel.—The improver of the thermometer and
barometer, born 1686 at Danzig. He used mercury instead of spirit for
thermometers and marked the freezing point of water at 32°, and the boiling
point of water at 212°.
The Fahrenheit scale is still extensively used in English-speaking countries.
The size of the degree is convenient and the range of the scale, 0° F. to 100° F.,
is a serviceable one for the climates of the temperate zone, where temperatures
below 0° F. at the earth's surface are of rare occurrence.
Fall.—" The fall of the leaf," a term in common use in America for
autumn.
Falling Time, Of Barometer.—When a mercurial barometer is raised quickly
from an inclined position to the vertical, the mercury will not fall immediately
to the equilibrium position appropriate to the pressure of the surrounding air.
The rate at which the mercury falls is for a particular barometer proportional
to h, where h is the height of the mercury above the equilibrium position, and
this rate is a measure of the sensitivity of that barometer. In marine
barometers the sensitivity is reduced, by constricting the tube, to minimise
PUMPING and it is specified that the falling time between heights corresponding
with pressures 50 and 18 mb. above the true pressure of the surrounding air
must lie between 4 and 5 minutes. The falling time so measured is equal to
the " lagging time " L, and when pressure is changing at a steady rate for a
period of many minutes the error due to lag is eliminated by reading the
barometer L minutes after the time at which the pressure is required.
False Cirrus.—A name previously used for cirrus nothus. See CLOUDS,
CIRRUS.
Fata Morgana.—A complicated form of MIRAGE in which multiple images
of an object are produced ; these images are frequently elongated. It is
produced by several layers of air of different refractive indices.
Fiducial Temperature.—The temperature at which the readings of a
barometer graduated in millibars need no correction in a particular latitude.
The maker of a millibar barometer aims at making it read accurately through­
out the scale at a certain temperature in lat. 45°. The temperature now
adopted is 285° A. This temperature, appropriate for lat. 45°, is called the
STANDARD TEMPERATURE of the barometer. Owing to the variation of gravity
with latitude a mercury column of given length causes a pressure which is
not invariable over the earth's surface but is dependent upon the latitude.
A barometer which reads correctly at 285° A. in lat. 45° does not, therefore,
read correctly at that temperature in another latitude. A temperature can,
however, be found for each latitude at which the readings will be correct.
This is termed the fiducial temperature. As an example, a barometer with
standard temperature 285° A. will have a fiducial temperature in lat. 51°
of 288° A.
Firn.—A German word meaning old snow which is in the process of being
transformed into glacier ice. The word is also used to denote the accumulation
area of old snow above a glacier and is synonymous with the French word
neV6. There is no corresponding word in English. G. Seligman suggests
that in English writings the term " firn snow " be used for the snow particles
which are in the process of transformation into ice, and " neve " to indicate
the accumulation area above a glacier. The changes which take place in
the newly fallen snowflake before it becomes glacier ice are described by
G. Seligman in " Snow Structure and Ski Fields " (Macmillan, London, 1936),
where a detailed nomenclature of the different snow forms is also given.
Floe.—An area of ice other than fast ice whose limits are within sight.
Floes up to 2 ft. in thickness may for convenience of description be termed
FOG
88
"' light floes " ; floes thicker than this, " heavy floes." Fast ice is sea ice
remaining in the position of growth and is only met along coasts where it
is attached to the shore or over shoals where it may be held in position by
islands or stranded icebergs.
Fog.—Fog is denned as obscurity in the surface layers of the atmosphere
caused by particles of condensed moisture or of smoke held in suspension in
the air. In international meteorological practice the term " fog " is limited
to a condition of atmospheric obscurity in which objects at a distance of
one kilometre are not visible. When visibility exceeds this limit, but is less
than two kilometres, the obscuration is called " mist " or " haze " according
to whether it is produced by condensed water particles or by foreign solid
matter such as dust or smoke.
If the accumulation of dust or smoke in the atmosphere is so great as to
reduce visibility below one kilometre, a pure dust fog or smoke fog occurs.
Smoke fogs are characteristic of large towns and industrial areas where there
is a continuous output of smoke and other impurities from factory chimneys,
etc. The worst type of fog in such areas occurs when conditions are also
favourable for condensation and a mixture of the two kinds of fog produces
considerable obscurity.
Dust fogs are associated with the desert regions of the globe. They occur,
for example, off the west coast of Africa during the season of the HARMATTAN
which carries clouds of dust from the Sahara.
The following scale of fog intensity is used in meteorological work :—
Symbol
F
f
..
m or z
..
Description
Dense fog
<T Thick fog
Fog ..
/ Moderate fog
Mist or haze
Limit of visibility.
Objects not visible at a
distance of
m.
yd.
40
44
200
220
400
440
1,000
1,100
2,000
2,200
Condensation of water vapour in the surface layers of the atmosphere
is brought about most frequently by the direct cooling of the air below its
dew point. The cooling can occur either by lowering of the temperature of
the surface of the ground, which is subsequently communicated to the air
above it, or by the drift of air over a surface which is colder than itself.
In both cases an inversion of the temperature lapse rate is formed in the
lower layers of air, and condensation is produced by the turbulent mixing
of the air within these layers (see TURBULENCE) . For the effective formation
of fog, the wind must be light, in order to allow the air in contact with the
ground to become sufficiently cooled. When fog has formed in the manner
outlined above, radiation from its upper surface produces further cooling and
so leads to an intensification of the fog.
Fogs over land occur chiefly in autumn and winter. They are formed
most frequently on the calm, clear nights which are associated with anticyclonic conditions. They reach their maximum intensity, normally in the
early morning, between one and two hours after sunrise, and usually disperse
before midday. In winter, however, such fogs occasionally cover a wide area
and persist for some days. Fogs may also be formed by the passage of warm
air over cold ground or by the mixing of two currents of air of different
temperatures. A peculiarity of the London area is the formation of fog, due to
condensation together with an accumulation of smoke, in a layer of air
above the surface. Such fogs, which are usually of brief duration, may cause
darkness in the middle of the day equal to that of night.
Fogs at sea, unlike land fogs, are characteristic of spring and summer,
and are usually formed by the passage of a current of air from a large land
mass, or from tropical or sub-tropical regions, over the relatively cold sea.
High-level country experiences fog at all seasons due to drifting low cloud
which envelops the high ground. High ground near the sea suffers the most
in this respect.
FOG
89
The height to which an ordinary fog extends varies considerably. It is
usually less than 1,000 ft. and frequently less than 500 ft. Sea fogs are
sometimes so shallow that the mast-heads of ships protrude above them.
In certain conditions a fog has no clearly denned upper boundary but merges
into cloud which may extend to a considerable height.
The following table shows the percentage frequencies of fog and mist at
different seasons at stations in the British Isles. The figures are based on
observations at 7h., 13h. and 18h. G.M.T. during six years, the frequencies
at each of these three hours being averaged to give the frequencies of whole
days of fog.
Number of days per 100
c/i
S
Station
Visibilities less than 1,000 m. Visibilities less than 2,000 m.
^
1j3 •»•!
.£?a
D-a
ffi
•a
00
lH
<D
1-1(D
00
%
a
G
o.
a
J>
<
1
C/3
a
&
_3
•g
H
<u
•4-*
-M
3
1
Valentia
Matin Head
Stornoway
30
51
30
0-1
0-6
0-5
0-3
0-6
0-1
0-1
0-6
0-9
0-0
0-4
0-1
0-1
0-8
1-1
0-5
1-2
1-4
0-4
1-0
1-5
0-2
1-0
0-6
Wick ..
Donaghadee
Mount Batten
(Plymouth)
Portland Bill
81
40
82
1-6
0-9
0-7
3-2
1-3
1-3
0-5
0-8
1-5
0-2
0-8
0-7
2-2
1-8
1-0
3-7
2-5
2-4
0-9
1-8
2-5
0-4
1-6
2-0
32
1-8
1-8
0-9
0-8
2-5
2-8
1-6
1-7
Holyhead
Lerwick
Leuchars
Falmouth
26
54
40
200
1-2
1-4
1-9
2-1
2-1
3-2
0-8
2-7
0-7
2-9
1-5
2-2
0-6
0-4
1-0
0-7
2-0
1-9
3-2
3-1
3-6
4-0
1-4
3-4
1-8
3-4
2-7
3-0
2-0
0-8
3-4
1-6
Dungeness
Calshot (Southampton)
Inchkeith
Gorleston
20
10
190
14
1-2
1-2
2-4
1-6
1-3
0-4
2-1
1-2
2-4
2-4
2-8
2-9
3-5
3-1
2-1
5-9
2-1
2-6
3-8
1-9
1-9
1-2
2-9
1-5
4-0
4-7
3-5
5-0
5-5
5-5
4-4
8-6
Roches Point.
Felixstowe
Scilly ..
Clacton
22
15
165
54
1-9
0-8
3-0
0-7
2-6
0-5
4-9
0-4
1-8
2-3
4-5
2-9
1-5
5-2
2-0
5-4
3-8
2-1
3-7
2-2
4.4
1-3
5-7
0-9
4-5
5-0
5-6
6-4
4-9
9-9
3-0
9-8
Aberdeen
Andover
Pembroke
46
295
150
2-2
2-5
4-6
1-6
0-9
4-8
2-0
3-3
2-7
1-7
6-5
4-1
5-3
3-7
5-6
3-1
1-7
6-0
4-9
5-9
3-6
6-4
9-4
5-2
Spurn Head
Cranwell
Shoeburyness
29
236
11
1-9
1-8
1-1
2-3
1-6
0-4
4-1
5-1
6-2
7-1
9-1
8-3
3-6
2-7
2-7
3-3
2-1
0-8
6-1
7-9
10-1
11-3
13-6
13-5
Ross on Wye
Birr Castle
223
173
1-7
1-6
1-2
1-0
7-6
5-4
5-1
5-1
3-7
5-8
2-2
2-7
13-5
9-2
10-0
14-2
Lympne
Tynemouth
Farnborough
350
67
230
2-7
3-3
3-2
2-1
4-2
0-7
4-6
5-0
8-1
10-0
6-2
7-3
5-0
6-3
5-5
3-6
5-9
1-3
8-6
9-7
14-3
16-2
12-0
12-6
Biggin Hill
Croydon
597
244
3-5
2-7
1-4
0-7
6-8
7-2
10-6
7-3
7-9
7-1
3-2
2-0
12-2
16-5
17-3
17-6
Liverpool
Sealand (Chester)
Kew
..
Renfrew
Birmingham
189
16
18
36
535
4-8
2-8
3-5
2-9
5-1
1-0
0-8
0-8
0-7
1-8
7-3
10-0
11-4
10-5
10-1
5-6
8-3
11-3
9-2
12-9
14-5
10-0
8-6
11-3
12-9
4-0
3-5
2-2
4-5
4-9
15-2
20-4
20-3
18-8
19-4
14-5
17-9
21-3
20-4
24-1
Beachy Head . .
525
10-1
9-5
8-2
17-1
15-1
13-4
18-2
24-6
FOG Bow
90
Fog Bow.—A white rainbow of about 40° radius seen opposite the sun
in fog. Its outer margin has a reddish and its inner a bluish tinge but the
middle of the band is quite white. A supernumerary bow is sometimes seen
inside the first and with the colours reversed. The bows are produced in the
same way as the ordinary rainbow but owing to the smallness of the drops,
the diameter of which is about 0-05 mm., the colours overlap and the bow
appears white.
Fohn.—A warm, dry wind which blows down the slopes on the leeward
side of a ridge of mountains. The name originated in the Alps where the
fohn is very prevalent, especially on the northern slopes in the inner valleys ;
the wind also occurs on the eastern side of the Rocky Mountains where it is
known as the CHINOOK. Owing to its warmth and dryness it melts the snow
very quickly and causes a considerable rise in temperature. It is frequently
accompanied by lenticular clouds.
The fohn occurs in the Alps mostly with a southerly wind. The air when
coming against the ridge of mountains ascends and dynamic cooling takes
place with condensation and precipitation. The temperature of the air falls
at the adiabatic rate for wet air, that is at about 1° C. for every 200m. of
ascent. When the air descends the slopes on the other side of the mountain,
having lost its moisture it is dynamically warmed at the " dry adiabatic "
rate of 1° C. for every 100 m. and reaches the valleys as a warm, dry wind.
The fohn is likely to occur wherever cyclonic systems pass over mountainous
regions ; for example it frequently occurs on the coast of Greenland where it
has a considerable influence on the winter climate.
Foot-pound-second System.—A system of units based upon the foot, the
pound and the second as fundamental units. Seldom used in meteorology
or the pure sciences, but forms the basis of the units used by engineers. On
this system the unit of force is the " poundal," which is the force required
to give a mass of 1 pound an acceleration of 1 foot per second per second.
The unit of work, the " foot-pound," is the work done in raising a weight of
1 pound through a vertical distance of 1 foot against gravity.
The foot-pound-second system is the counterpart of the C.G.S. system,
in which the fundamental units are the centimetre, gram and second.
Forecast.—The first use in this country of synoptic charts for forecasting
the weather anticipated in the near future was made by Admiral Fitzroy in
1860, and it was he who invented the special meaning of the term " forecast "
to avoid the somewhat unfortunate connotations attaching to such terms as
" prognostic " and " prophecy."
In practice, a forecast as issued, for 12, 24 or 36 hours, includes :—
(1) A statement of the anticipated direction and force of the surface
wind and the changes therein which are expected during the period of
the forecast.
(2) A statement of the anticipated state of the sky (as regards clouds),
precipitation (rain, hail, snow or sleet) and temperature, whether it is
likely to be greater than, equal to or less than, the normal for the time
of year, or higher or lower than at the time of making the forecast.
(3) A note as to the probability of such occurrences as night frost,
fog or thunder.
Forecasts issued for special purposes contain special matter, additional to
that already specified. For aviation, for example, the anticipated winds at
selected or demanded heights are included together with a statement
concerning the visibility and any anticipated changes in it. Again, for
seamen, statements may be included concerning visibility and the state of
the sea.
To derive the forecasts, the forecaster depends upon a close study of the
current synoptic charts, especially with regard to anticipated changes in the
distribution of pressure, and with constant appeal to physics and to precedent.
91
FREQUENCY
The period covered by the forecasts issued by the Meteorological Office
does not exceed, as a rule, 24 to 36 hours, but usually a statement under the
heading FURTHER OUTLOOK is added, giving the conditions likely to be
experienced in the 24 hours or more following the period covered by the
actual forecast. Attempts to find means to forecast the coming of weather
changes at a greater distance ahead are described under LONG-RANGE FORE­
CAST. See also SINGLE OBSERVER FORECASTING.
Fortin Barometer.—A portable form of mercurial BAROMETER in which
the zero of the scale is fixed by a pointer, inside the cistern. By making the
cistern partly of leather and providing an adjusting screw, the level of mercury
in the cistern can be brought up to the scale zero before each reading is taken.
Fourier Series.—A representation of any periodic function of an inde­
pendent variable in terms of sines and cosines of multiples of that variable is
called a Fourier Series. It was first developed by Fourier in his treatise
" Theorie de la Chaleur " (Paris, 1822). In symbols,
f (x) = A 0 + A x sin x + A 2 sin 2 x +
....
+ Bj cos x + B2 cos 2 x +
See HARMONIC ANALYSIS.
Fracto.—An adjective used in cloud nomenclature to denote a ragged
broken structure (fractocumulus, fractostratus, fractonimbus).
Frazil Ice.—Ice which forms in spicules or small plates in rapidly flowing
rivers, the movement of the water preventing the ice crystals from forming
a solid sheet of ice. The formation has been best observed in the rivers of
Canada, and the name is a French-Canadian word, from a French word for
cinders, the frazil crystals being supposed to resemble forge-cinders.
Freeze, Freezing.—With reference to the weather, " freezing " is used
when the temperature of the air is below the freezing point of water. In
America, freezing conditions sufficiently persistent to produce what we should
call " a FROST " are usually described as " a freeze."
Frequency.—The number of times that a particular phenomenon of
weather has happened in the course of a given period of time, generally a
Frequency Table.—Number of Spells of Wind of Specified Duration from NE. t
E. or SE., during the nine years 1904-12 inclusive. South-east England
and northern France.
n
Duration of Spell
x
u
IH
03
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XI
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Days
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2
3
4
5
6
7
8
9
10
11
12
13
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5
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No. No. No.
7 13
12
3
3
7
5
4
2
3
3 —
—
1 —
1 — —
— —
1
1 — —
— — —
— — —
—
1 —
— — —
— —
1
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No.
13
6
4
4
—
—
—
—
—
—
—
—
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No.
13
5
—
2
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2
—
—
—
—
—
1
—
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No. No. No.
15 15 19
7
3 10
3
3
3
1
1
1
2
4 —
1
2
2
1
1 —
1 — —
— — —
— — —
— — —
— — —
— — —
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No.
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8
3
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1
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—
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—
—
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No.
12
4
7
1
1
—
-—
2
1
—
—
—
—
No.
13
4
1
2
1
1
—
—
—
1
—
—
—
No.
11
2
2
—
—
1
1
1
—
_
—
—
—
1
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Total number of days of E. 58 44 63 71 74 65 53 55 76 75 53 42
wind.
Total number of days of 279 255 279 270 279 270 279 279 270 279 270 279
observations.
FRICTION
92
number of years. Here, for example, is a summary of the spells of wind from
the easterly quarter, according to the direction of the isobars, over south­
east England and northern France in nine years. Taking January, for instance,
the nine years supply a total of 279 days, of which 58 were days of E. wind.
These consisted of one sequence of eight consecutive days of E. wind, one
sequence of six days, three sequences of four, two sequences of three and
seven sequences of two days, with finally twelve isolated days of E. wind.
In this case the number of years over which the observations extend is
given, quite an arbitrary number, and thus the numbers for frequency of
occurrence have to be considered with reference to the number of years
selected. It is, however, usual to reduce frequency figures to a yearly average.
In view of the awkwardness of having to bear in mind the possible number
of occurrences, while considering the actual or average number, it is
convenient to use the percentage frequency instead of the actual frequency.
This plan is often adopted for giving the results of observations at sea, which
are made six times a day, or every four hours.
It is often convenient to represent the results of observation by means of
a frequency curve. An example is shown giving the distribution of frequencies
of deviations from normal of monthly mean pressures at Edinburgh for the
period 1770-1869. (Fig. 19.) This curve approximates to the "normal
frequency curve " given by the NORMAL LAW (OF ERRORS).
, 160
EDINBURGH PRESSURES
'70-IK 9.
JCY
Cl
fRCQUEl
UlpVE
T
o
100
_so
\
60
•40
•30
20
•10
0
FIG. 19.
10
•20
•30
•40
•50
Friction.—A word used somewhat vaguely in meteorological writings in
dealing with the effect of the surface of the sea or of the land, with its
obstacles in the form of irregularity of surface, hills, buildings, or trees upon
the flow of air in the lower layers of the atmosphere. The effect of the
irregularities of surface is to produce TURBULENCE in the lowest layer which
gradually spreads upwards, if the wind goes on blowing, and consists of
irregular eddies approaching to regularity in the case of a cliff eddy which
can be noticed when a strong wind blows directly on to a cliff and produces
an eddy with a horizontal axis. An account of the eddies caused by the eastern
face of the rock of Gibraltar is given in Geophysical Memoirs, Vol. VII, No. 59,
1933.
93
FRONTOGENESIS
The general effect of this so-called friction is to reduce the flow of air
past an anemometer so that the recorded wind velocity is below that which
would be experienced if the anemometer were high enough to be out of the
reach of the surface effect. Numerical values for this effect are of great
practical importance, because they are concerned with the change of velocity
in the immediate neighbourhood of the ground. But it is not easy to obtain
them, because every exposure near land or sea is more or less affected and
therefore no proper standard of reference can be obtained by direct
observation. Recourse is, therefore, had to the computation of the wind from
the distribution of pressure, the so-called " geostrophic " or GRADIENT WIND.
From the comparison of a long series of geostrophic and observed winds
we conclude that over the open sea, or on an exposed spit of flat sand like
Spurn Head, the wind loses one-third of its velocity from " friction," and at
other well-exposed stations the loss is, on the average, as much as 60 per
cent., but for any particular anemometer it is different for winds from different
quarters because the exposure seaward or landward is different. Information
on this point for a number of Meteorological Office stations is given in a
memoir by Mr. J. Fairgrieve (Geophysical Memoirs, Vol. 1, No. 9, p. 189).
The consequence of this effect can sometimes be seen in weather maps.
On one occasion when the whole of the British Isles was covered with parallel
isobars running nearly west and east, all the stations on the western side
gave the wind as force 8 (42 mi./hr.), while those on the eastern side gave
force 5 (21 mi./hr.), so that the velocity was reduced by one-half in consequence
of the " friction " of the land. If the velocity at the exposed western stations
be taken at two-thirds the velocity of the wind free from friction, we get the
following interesting result which is probably correct enough for practical
use :—One-third of the velocity is lost by the sea friction on the western
side, and one-third more by the land friction of the country between west
and east.
Front.—The term introduced by the Norwegian meteorologists to denote
the line of separation between cold and warm masses of air. The more
important fronts are due primarily to the large-scale horizontal movements,
which bring masses of air of widely different origin into juxtaposition. The
actual boundary is sometimes diffuse, and only becomes sharp when there is
pronounced CONVERGENCE (see FRONTOGENESIS) . Sharp fronts are often made
diffuse by SUBSIDENCE in the cold air, provided that there is little precipitation,
the cold air behind the front descending and being warmed by compression
so that it becomes difficult to distinguish it from the warm air which it is
displacing. From the line of the front on the ground, a SURFACE OF DIS­
CONTINUITY or frontal surface slopes upward over the cold air. The contrast
of temperature, and therefore of density, affects the pressure and wind
distribution, and it can be shown that a front must in general lie along a
trough of low pressure (though a rounded trough is not necessarily a front).
It may, however, be parallel to the isobars, in which case the steeper pressure
gradient is on the side of the higher pressure. In consequence, surface friction
normally causes some convergence and rainfall along a front, but large
amounts of rain can only occur when there is convergence extending up to
some thousands of feet, which results in the warm current rising en masse
up the sloping frontal surface. See also DEPRESSION, POLAR FRONT, COLD
FRONT, WARM FRONT, AIR MASS.
Frontogenesis.—The development or marked intensification of a FRONT.
The process is really a development from a zone of transition between AIR
MASSES, indicated by a steep horizontal gradient of temperature, to a real
DISCONTINUITY. The drawing together of the isotherms is accompanied by
the removal of the intermediate air, either horizontally (deformation) or
vertically; the latter involves CONVERGENCE and is the most potent factor.
Suitable surface conditions, such as coast lines, snow or ice boundaries, or
abnormal gradients of sea temperature, favour frontogenesis.
FRONTOLYSIS
94
Frontolysis.—The disappearance or marked weakening of a FRONT. The
opposite of FRONTOGENESIS. SUBSIDENCE is the most important factor in
causing frontolysis.
Frost.—Frost occurs when the temperature of the air is below the
freezing point of water. It may be transient and local, as is usual in the
case of frosts due to the cooling of comparatively mild air of oceanic origin
at night under a clear sky, or it may continue night and day over a large
area. When persistent frost occurs over a large proportion of the British
Isles, it is always initiated by the arrival of dry cold air from the continent
or the Arctic Regions. A persistent anticyclone to the north-west, north
or north-east of the British Isles in the winter half of the year, may cause
such an influx of cold dry air, generally with northerly or easterly winds.
If these bring heavy snow, the spell of frost is apt to be severe, because snow
absorbs only a small proportion of the solar radiation that falls upon it, while
emitting radiation of longer wave-lengths very readily; and because its
thermal conductivity is so low compared with that of the ground that the
normal winter flow of heat from the deep earth to the surface is checked.
It is with snow-covered ground that temperatures below zero Fahrenheit
are liable to occur even in the south of England. The exceptional frosts of
February, 1895, were preceded by heavy snow. The expression " degrees
of frost " denotes the number of degrees that the temperature falls below the
freezing point.
Frost Day.—A frost day is defined as a period of 24 hours beginning
normally at 9 h. G.M.T. on which the screen minimum temperature is 32° F.
or less.
Frost, Protection against.—The use of orchard heaters for the purpose of
protecting fruit crops from destruction by frost during the blossoming season
has obtained a considerable vogue in the United States of America. In this
country severe frost during the period of maximum susceptibility is relatively
rare, but the American methods of orchard heating have been adopted by
some growers. Accurate data are scanty, but it appears that it is practicable,
without a prohibitive expenditure of fuel, to maintain the orchard temperature
above the lethal point, on " radiation nights " when there is no general air
movement and frost is the result of nocturnal radiation from the ground and
plants. In these circumstances the heating effect is applied to the cold surface
layers of air beneath the inversion, and an appreciable rise of temperature
may be produced. In a "wind frost," on the other hand, when a lethal
temperature is due to a definite flow of cold air of great vertical extent, no
process of heating could ever be effective. Some American growers have also
used large fans rotated about an inclined axis by powerful motors supported
on towers thirty or forty feet high. The aim is to draw down warm air from
higher levels to displace the cold surface layers of air, and a high standard of
efficiency is claimed by the makers of the installations.
Funnel Cloud.—The cloud formed at the core of a WATERSPOUT or TORNADO,
sometimes extending right down to the earth's surface, attributed to the
reduction of pressure at the centre of the vortex.
Further Outlook.—A statement in brief and general terms appended to a
detailed forecast and giving the conditions likely to be experienced in the
24 hours or more following the period covered by the actual forecast.
Gale.—As ordinarily understood a gale simply means a high wind. For
technical purposes however it is necessary to have a more definite specification,
and it is now the practice in the London Meteorological Office to limit the
term gale to winds of force 8 or above on the BEAUFORT SCALE, although
Admiral Beaufort himself described force 7 as a moderate gale. As the
result of international agreement it has been decided that the equivalent
velocity of force 8 on the Beaufort scale extends from 15-3 to 18-2 metres
per second for an anemometer exposed at a height of 6 metres over a level
surface, this corresponds to velocities between 17-2 and 20 • 7 metres per second
for an exposure 10 metres above the ground, which is the normal exposure of
GALE WARNING
95
the anemometers used at British stations. Great difficulty is encountered
in comparing the frequency with which gales occur at different places. If
the observations to be compared are all made by estimates according to the
Beaufort scale, the comparison is made difficult because of the different
exposure to the wind at the place of observation. Observations made within
a sheltered harbour will obviously give fewer gales than observations made on
a nearby headland. Similar difficulties as regards the exposure occur with
records obtained by anemometers ; but in addition there are difficulties due
to the varying height of the anemometers above the ground and also due to
the method of reading the anemometer trace. The velocity equivalent of
17-2 m./sec. was determined from records each extending over an hour
during which the velocity was frequently above this value and also
frequently below. Thus it is not correct to report a gale whenever an anemo­
meter trace reaches 17-2 m./sec. From this it is clear that it is essential to
state exactly how statistics of frequency of gales have been obtained, and
to take into account when discussing the statistics the " exposure " at
the observing stations.
The table headed " Average occurrence of gales during the 20 years
1916-35 " that is appended to this article is based on as homogeneous a set
of statistics as it is possible to obtain. The majority of the observations are
" Beaufort estimates," which were made on lightships and lighthouses in
exposed situations and coastguard stations, 1916-35. The table shows that
the south-west of England is the most subject to gales and the east of
Scotland the least subject.
Average occurrence of gales during the 20 years 1916-35
Districts
Scotland :—
North-east
East
North-west
West and North
Channel.
Ireland : —
North
South
Irish Sea
St. George's Channel
Bristol Channel
England : —
South-west
South
South-east
East
North-east
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5-5
4-8
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1-8
3-6
2-9
2-3
0-7
2-4
1-9
1-7
0-6
1-1
1-3
0-5
0-3
0-5
0-4
0-5
0-3
0-5
0-5
0-1
0-1
0-2
0-2
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3-7
3-5
1-4
3-7
3-6
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1-9
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4-2
28-4
11-9
27-4
24-9
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4-7
4-5
4-7
5-1
3-8
3-0
2-6
2-5
3-3
1-9
2-1
2-1
1-7
2-4
1-5
1-7
1-5
1-3
1-6
0-5
0-3
0-6
0-4
0-9
0-6
0-4
0-7
0-4
0-7
0-3
0-3
0-3
0-3
0-6
0-3
0-8
0-5
0-7
1-1
1-9
1-5
1-9
1-7
1-7
3-5
3-7
3-9
3-5
4-7
3-7
3-9
3-0
3-6
4-7
3-9
4-4
4-1
4-0
4-9
28-3
26-8
25-5
24-7
31-5
5-3
4-1
3-7
3-1
2-3
3-5
2-5
2-5
2-7
2-1
3-1
1-7
1-7
1-8
1-2
2-3
1-5
1-6
1-6
0-7
0-8
0-3
0-5
0-5
0-1
0-7
0-2
0-4
0-4
0-1
0-5
0-5
0-7
0-5
0-1
0-9
0-7
0-7
0-8
0-3
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1-3
1-5
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4.4
3-9
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6-1
4-1
4-1
3-2
2-1
34-5
24-5
24-7
23-1
13-5
Gale, Day of.—In British climatology a day of gale is denned as a day on
which a wind of force 8 or more (BEAUFORT SCALE) has lasted for at least
10 minutes at any time during the 24 hours ending at midnight G.M.T.
Gale Warning.—The Meteorological Office issues notice of the probability
of gales on or near the coasts of the British Isles by broadcast warnings and
by telegram to ports and fishing stations recommended by responsible local
authorities, and the fact that one of these notices has been received at any
station is made known by hoisting a cone, 3 ft. high and 3 ft. wide at
base, and painted black.
Two cones are used (a) the south cone (point downwards) and (b) the
north cone (point upwards). For the significance of these cones reference
should be made to the instructions issued to^all gale-warning stations.
GAS
96
The issue of a warning indicates that an atmospheric disturbance is
moving or developing in such a way as probably to cause a gale in exposed
situations or in the open sea in the district to which the warning is sent, i.e.
within a distance of 50 to 100 miles of the place where the cone is hoisted.
The meaning of the warning is simply " Look out for high winds or gales."
It needs to be noted that, in connexion with the gale warning service,
the term " gale " is used for winds of force 8 and above on the BEAUFORT
SCALE OF WIND FORCE.
Gas.—The name used for any kind of fluid which has unlimited capacity
for expansion under diminishing pressure. It is to be distinguished from a
liquid which has only a limited capacity for expansion under reduced pressure.
A liquid may occupy only the lower part of a vessel like a bottle ; it
will flow to the bottom of the vessel and leave a " free " surface. But a
gas cannot be located in that way ; its volume is determined not by the
amount of material but by the size of the vessel which contains it and by
the pressure upon its boundaries.
In scientific practice gas means any substance which obeys approximately
the gaseous laws ; these laws are two, viz. :—
(1) When the temperature is kept constant the pressure of a given
mass of gas is inversely proportional to the volume which it occupies,
or the density is directly proportional to the pressure.
(2) When the volume is kept constant the pressure is proportional to
the ABSOLUTE TEMPERATURE, or when the pressure is kept constant the
volume is proportional to the absolute temperature.
Gathering Ground.—An area from which water is obtained by way of
rainfall, drainage or percolation. See also CATCHMENT AREA and DRAINAGE
AREA.
Gauss.—See ELECTRICAL UNITS.
General Inference.—The term " General Inference " is used in weather
forecasting for a description of the general pressure distribution and the
changes of pressure in progress together with a statement of the type of
weather likely to be experienced through such changes. It usually precedes
a series of more detailed forecasts for individual districts and gives the
foundations or framework on which these forecasts are formed.
Geophysics.—Physics is a comprehensive name for the branches of science
which deal with those natural phenomena which are not biological and not
merely descriptive. Geophysics is the name for those parts of physics which
are concerned with the earth and its atmosphere. Meteorology, seismology,
terrestrial magnetism, atmospheric electricity and hydrology (including the
theory of the tides) all rank as geophysical subjects. Geology and geography
are concerned largely with matters which do not come within the range of
geophysics, but there is no definite limitation.
Geopotential.—Potential energy is the general name given to ENERGY
which is available for conversion to kinetic energy. Thus we may talk of
the potential energy of a coiled-up spring. In many cases the potential
energy is acquired by a change of position in a field of force and in such cases
potential is defined as the potential acquired by the appropriate unit of
quantity. When dealing with matter in the gravitational field of the earth,
potential, or more specifically geopotential is the potential energy of unit
mass. The zero of potential is taken as at sea level.
Since the gravitational attraction of the earth is greater near the poles
than near the equator more energy is required to raise a body to a given
height near the poles, and therefore the geopotential increases more rapidly
as we ascend in high latitudes than in low. Conversely the height at which
a given potential is reached is less in high latitudes than in low. Points
with the same geopotential may be said to be at the same level. By using
geopotential rather than height for specifying the position of parts of the
97
GOLD SLIDE
atmosphere, the consideration of the air movements is simplified, and for
this reason the practice of publishing the results of balloon soundings in terms
of geopotential has been advocated and indeed adopted by the international
organization.
The natural unit of geopotential is the potential acquired when a mass
is raised through the unit distance in a field of force of unit strength. Thus,
in the C.G.S. system the unit would be 1 cm. x 1 cm ./sec. 2 The unit advocated
by Bjerknes is 105 times greater and is known as the " dynamic metre " ;
though some meteorologists regard this term as objectionable in that it
confounds geopotential with height.
General Rainfall.—The mean value of the depth of rainfall for a given period
over an area of substantial extent.
Geostrophic.—See GRADIENT WIND.
Glacier Breeze.—A cold breeze, blowing down the course of a glacier,
which owes its origin to the cooling of the air in contact with the ice. See
KATABATIC.
Glaisher Stand.—A form of stand devised by James Glaisher for the
exposure of thermometers. The stand consists of a vertical portion, partially
roofed, on which the thermometers are mounted, with a doubly roofed sloping
rear portion designed to prevent the front portion becoming heated from the
rear. The whole is capable of rotation about a vertical axis so that direct
sunshine may be prevented from affecting the thermometers at all times. The
stand has been superseded at nearly all important stations by the STEVENSON
SCREEN.
Glazed Frost.—When rain falls with the air temperature below the freezing
point a layer of smooth ice, which may attain considerable thickness, is
formed upon all objects exposed to it. This is known as glazed frost. The
accumulation of ice is frequently sufficient to bring down telegraph wires.
In these Islands the phenomenon is one of comparative rarity. It occurred
on the morning of December 21, 1927, in London, and in many other parts
of England, and caused several thousand street accidents. Glazed frost also
occurs when a warm damp wind supervenes upon severe cold, the moisture
condensing on still freezing surfaces and thus producing a coating of ice.
A phenomenon similar to glazed frost may also result if sleet, formed
by the passage of snow through an upper stratum of air above the freezingpoint, occurs during frost. Instances of this occurred in London during the
winter of 1916-7, and road traffic, in some cases even rail locomotion, was
rendered difficult or impossible.
Glory.—The system of coloured rings surrounding the shadow of the
observer's head on a bank of cloud or mist. A typical series of colours in a
well developed glory is the following :—A whitish yellow field bounded by
dull red ; bluish green, reddish violet; blue, green, red ; green, red.
When light passes through circular holes in an opaque screen, colours are
produced by DIFFRACTION. If little mirrors all facing the sun could be
substituted for the droplets in a cloud, the light from each mirror would behave
as if it came through a hole from the reflection of the sun and similar diffraction
colours would occur. The action of the drops is probably analogous. The
mathematical theory developed by Ray is on these lines. Earlier writers had
supposed that the phenomena were produced by the diffraction, by particles
comparatively near the surface, of light reflected from deeper portions of the
fog or cloud.
A glory may be surrounded by a FOG BOW.
Gold Slide.—When a mercurial barometer is used at sea level in lat. 45°
the correction to be applied to its readings in order to reduce them to standard
conditions is directly proportional to the difference between its STANDARD
TEMPERATURE and the actual temperature. To simplify the application of
this correction, it is clearly possible to graduate the attached thermometer
(90566)
D
GRADIENT
98
directly in millibars, the zero corresponding with the standard temperature.
If the height of the barometer cistern is changed by an amount not exceeding
100 ft. the consequent correction is merely equivalent (to a first approximation)
to a change in the zero of the correction scale. The same statement applies
to a change of latitude.
In the Gold slide, devised by Lieut.-Col. E. Gold, this principle is applied
in a practical form for use on marine barometers. The " ATTACHED
THERMOMETER " is mounted in a brass stock and the " barometer correction
scale " of millibars is mounted on a vertical slide actuated by rack and
pinion. Scales are provided whereby the necessary adjustments for index
error, height and latitude are readily made and the total correction is read
off on the sliding scale opposite the end of the thermometric column, thus
avoiding calculation or reference to tables.
Gradient.—The word gradient is used in surveying and in common
practice to indicate the slope of a hill, that is the change in height per unit
horizontal distance. The word has been adopted in meteorology to indicate
the change in certain elements, for example pressure and temperature, per
unit horizontal distance and its use has further been extended to the change
of elements in the vertical, though this use seems hardly justified. Thus,
when temperature gradient is referred to, it is generally the change of
temperature with height in the atmosphere which is meant and not the change
along the earth's surface between one place and another. More recently the
term LAPSE rate has been used to replace gradient in the vertical direction ;
thus, if the temperature falls at the rate of 5° per 1,000 ft. height in the
atmosphere, this is referred to as a lapse rate of 5° per 1,000 ft.
Of the several connexions in which the word gradient is used in meteorology
that in which it indicates the rate of change of pressure along the earth's
surface is the most important, pressure gradient being one of the fundamental
quantities with which the meteorologist has to deal. This is mainly due to
the fact that the wind at a short distance above the earth's surface is so closely
related to the pressure gradient that in many cases a better estimate can be
formed of the general wind from the run of the isobars than by any other
means. The pressure gradient is the change of pressure per unit distance
measured perpendicular to the isobars, that is in the direction in which pressure
changes most rapidly. Instead of measuring the pressure difference in a
given distance such as 100 miles, it is in practice found more convenient to
measure the distance between two consecutive isobars and deduce the gradient
by dividing the pressure difference between the two isobars by this distance.
The measurement of pressure gradient is a simple matter when the isobars
are straight or only slightly curved, are parallel to one another, and are
uniformly spaced. When the pressure distribution is irregular and the isobars
sinuous, the measurement of the pressure gradient becomes more difficult
and it is necessary to study the individual pressure readings in addition to the
run of the isobars to determine the gradient in any region.
Gradient Wind.—The flow of air which is necessary to balance the pressuregradient. The direction of the gradient wind is along the isobars, and the
velocity is so adjusted that there is equilibrium between the force pressing the
air inwards, towards the low pressure, and the centrifugal action to which the
moving air is subject in consequence of its motion.
In the case of the atmosphere the centrifugal action may be due to two
separate causes ; the first is the tendency of moving air to deviate from a
GREAT CIRCLE in consequence of the rotation of the earth ; the deviation is
towards the right of the air as it moves in the northern hemisphere, and
towards the left in the southern. The second is the centrifugal force of
rotation in a circle round a central point according to the well-known formula
for any spinning body. In this case we regard the air as spinning round an
axis through the centre of curvature of its path. This part of the centrifugal
action is due to the curvature of the path on the earth's surface. Both
components of the centrifugal action are in the line of the pressure gradient:
99
GRADIENT WIND
the part due to the rotation of the earth is always tending to the right in the
northern hemisphere, the part due to the curvature of the path goes against
the gradient from low to high when the curvature is cyclonic, and with the
gradient when it is anticyclonic, so that in the one case we have the gradient
balancing the sum of the components due to the earth's rotation and the spin,
and in the other case the gradient and the spin-component balance the action
due to the earth's rotation.
The formal reasoning which leads up to this result is given at the end of
this article. The two components have been named by Sir Napier Shaw—
that due to the rotation of the earth being called the geostrophic component
and that due to the curvature of the path, the cyclostrophic component.
Consider the relative magnitude of these components under different
conditions. It will be noticed that the geostrophic component depends upon
latitude, the cyclostrophic component does not, so, other things being equal,
their relative importance will depend upon the latitude; so we will take
three cases, one near the equator at latitude 10° within the equatorial belt
of low pressure, one near the pole latitude 80° of undetermined meteorological
character, and one, half-way between, in latitude 45°, a region of highs and
lows travelling eastward.
Using V to denote the wind velocity, when the radius of the path is
120 nautical miles the cyclostrophic component is equal to the geostrophic—
in latitude 10° when V is 5-6 metres per second;
in latitude 45° when F is 22-9 metres per second;
in latitude 80° when V is 31 -9 metres per second.
It will be seen that in the equatorial region the cyclostrophic component is
dominant as soon as the wind reaches a very moderate velocity.
For practical application in deducing the geostrophic component of the
gradient winds from isobaric charts a graduated scale is used, the graduations
being selected to correspond with the average latitude and average surface
conditions to which it is to be applied. If the distance between the isobars
for a definite value V of the geostrophic component is D for latitude 90°
ISO MILES
6O°
50*
2O*
(90565)
440
D2
GRADIENT WIND
100
then the distances between the isobars for any other latitude A for the
same value V of the wind is D cosecA.
If "the isobars are drawn for
intervals of 2 mb. and V is 10 mi./hr. then the value of D is 150 miles. The
diagram on p. 97 drawn to scale gives the actual distances apart of 2 mb.
isobars on a map scale 1 : 107 for different latitudes. The distances given
are for air of standard density, i.e. 0-001226 gm./cm.s. They are the actual
lengths of the horizontal line drawn at each latitude. The distances in miles
are given at the right of the diagram. (For other distances or densities,
speeds are inversely proportional, i.e. the simple rule is that halving the
distance or the density doubles the speed.)
EQUATION FOR GEOSTROPHIC WIND.
The Relation between the Earth's Rotation and the Pressure Distribution for
Great-Circle-Motion of Air.
The rotation to of the earth about the polar axis can be resolved into
eo sin <}) about the vertical at the place where latitude is <{> and to cos $ about
a line through the earth's centre parallel to the tangent line.
The latter produces no effect in deviating an air current any more than
the polar rotation does on a current at the equator.
The former corresponds with the rotation of the earth's surface counter­
clockwise in the northern hemisphere and clockwise in the southern hemisphere
under the moving air with an angular velocity to sin (}>. We therefore regard
the surface over which the wind is moving as a flat disc rotating with an angular
velocity to sin (j>.
By the end of an interval t the air will have travelled Vt, where V is the
wind velocity, and the earth underneath its new position will be at a distance
Vt x to / sin (j), measured along a small circle,
from its position at the beginning of the time t.
Taking it to be at right angles to the path, in the limit when t is small,
the distance the air will appear to have become displaced to the right, over
the earth, is V to tz sin (j>.
This displacement on the " £ gtz " law (since initially there was no transverse
velocity) is what would be produced by a transverse acceleration
2toFsin<j>.
Hence the effect of the earth's rotation is equivalent to an acceleration
2toFsin(j>, at right angles to the path directed to the right in the northern
hemisphere, and to the left in the southern hemisphere.
In order to keep the air on the great circle, a force corresponding with
an equal but oppositely directed acceleration is necessary. This force is
supplied by the pressure distribution.
EQUATION FOR CYCLOSTROPHIC WIND
Force necessary to balance the Acceleration of Air moving uniformly in a small
circle, assuming the earth is not rotating
Let A be the pole of circle PRQ. Join PQ, cutting the radius OA in N.
Acceleration of particle moving uniformly along the small circle with velocity V
is VZ/PN along PN = VZ/R sin p where R = radius of earth ; and p is the
angular radius of the small circle representing the path.
101
GRAUPEL
The horizontal component of this acceleration, that is, the component
along the tangent at P, is F2 cos p/R sin p = F2 cot p/R.
GENERAL EQUATION CONNECTING PRESSURE GRADIENT, EARTH'S
ROTATION, CURVATURE OF PATH OF AIR AND WIND VELOCITY
I. Cyclonic motion. —The force required to keep the air moving on a great
circle in spite of the rotation of the earth must be such as to give an acceleration
2o)Fsin<|> directed over the path to the left in the northern hemisphere. It
must also compensate an acceleration due to the curvature of the path,
V2 cot p/R, by a force directed towards the low pressure side of the isobar.
For steady motion these two combined are equivalent to the acceleration
y where D is the density of the air, and
due to the gradient of pressure, i.e. yr
•y the pressure gradient, directed towards the low pressure side,
V*
= = 2 CD V sin
+ p cot p.
———•
II. Anticyclonic motion.—In this case 2et>Fsin(j) and y/D are directed
outwards from the region of high pressure, and the equation becomes
y
F2
£ = 2 co Fsin<|> — -= cot p.
The cyclostrophic component does not represent the whole of the accele­
ration, there is also the acceleration along the path of the air, which is generally
of the same order of magnitude as the acceleration at right angles to the path.
Gram.—The unit of mass in the C.G.S. system. It is one-thousandth
part of the standard kilogram which was originally supposed to represent
the weight of a cubic decimetre of pure water at 4° C. ; but subsequent
research has shown that the relationship was not exact. A gram= 15• 4 grains,
or rather more than ^ ounce.
Gram-Calorie.—See CALORIE.
Grass Minimum Temperature.—The minimum temperature indicated by a
thermometer freely exposed in an open situation at night with its bulb if.
contact with the tips of the grass blades on an area covered with short turn
If under these conditions the minimum reading is 30-4°F. or lower, a
GROUND FROST is said to occur.
Graupel.—The German word for SOFT HAIL.
GRAVITY
102
Gravity.—The attraction between material bodies. The law of universal
gravitation is that every mass attracts every other mass with a force which
varies directly as the product of the attracting masses and inversely as the
square of the distance between them. It is convenient to regard the attracted
body as of unit mass. The law then implies that the force exerted is inde­
pendent of the temperature and of the velocity of the attracting body. Both
these conclusions have been attacked of late years, but it is not questioned
that they are sufficiently exact for meteorological purposes.
It was shown by Newton that a sphere whose density varies only with
the distance from the centre attracts an external body exactly as if the whole
mass were collected at the centre, and that a similarly constituted spherical
shell—i.e. a mass bounded by two concentric spherical surfaces—while
attracting an external body as if its mass were collected at the centre, exerts
no attraction at any internal point. Let us apply this to a point in the
atmosphere at height h above the ground, regarding the earth as a perfect
sphere of radius R, and assuming the density, whether of the earth or the
atmosphere, to vary only with the radial distance. The atmosphere outside
the spherical surface of radius R + h exerts no attraction, while the earth's
mass plus that of the atmosphere (M + M') within the surface of radius
R + h attract as if collected at the centre. Thus the attractive force is
G (M + M')I(R + h) 2, where G is a constant. This becomes g0 (I + M'/M)
(1 + h/R)- 2, where g0 = GM/R2 is the corresponding force at the earth's
surface, i.e. wholly within the atmosphere. Counted in kilograms M' is large,
but even if we went to the confines of the atmosphere M'/M would be less than
one millionth. Thus the attraction of the atmosphere may be neglected, at
least for meteorological purposes. The variation with height of the earth's
own attraction is much more important. At all heights attained by balloons
we may neglect h2/R2, and so replace (1 + h/R)~ 2 by 1 — 2h/R. But at a
height of 10 miles this represents a reduction of one part in 200 in gravity.
Although the astronomical definition of gravity relates only to the universal
attraction between masses, in geophysics the value of the gravitational
acceleration g is taken to represent the total vertical acceleration and
therefore to include the centrifugal acceleration.
In reality the earth is not a sphere but approaches to a spheroid whose
equatorial radius is 10-7 k. longer than its polar radius. The earth's surface
is also irregular in outline, and the density variable, at least near the surface.
Thus the formulae actually advanced to show the variation of gravity at
different parts of the earth's surface are complicated.
The value of g at mean sea level in lat. (J> is
g = 980-617 (1 - -00259 cos2<j>) in C.G.S. units.
where 980-617 cm./sec. 2 is the value of g at mean sea level in lat. 45°.
In the free air therefore, the value of g at height h in lat. & is given by
2A
g = 980-617 (1 - -00259 cos2<j>) (1 - -^-)
where R is the radius of the earth (R = 6,370,000 metres = 20,900,000 ft.).
The above formula would also hold at places on mountains if the effects
of the mountains were compensated by a change in the mean density of
the earth's crust beneath. If there were no such compensation, g would
be given by
5A
g = 980-617 (1 - -00259 cos2(j>) (1 This is the formula which is used in the International Tables for correcting
pressure at station level to standard gravity.
It will be seen that g has its mean value in lat. 45°, i.e. where cos 2<j>
vanishes.*
* At Berlin in 1 939 the Conference of Directors accepted 980 • 62 for g15, bu t at Washington
1947 the Conference decided to ask the International Association of Geodesy for advice on the
value. At present, 1949, the matter is under consideration.
103
GREGALE
This explains why it is usual to reduce gravity to lat. 45°. This means
reducing some measure actually made—e.g. of the height of the barometer—
to what it would have been if gravity had possessed its mean value. The
formula does not, of course, imply that gravity has the same value at every
spot in lat. 45°, irrespective of its height above sea level or other local
peculiarities.
The determination of g absolutely at any spot with the precision which
the formulae suggest is extremely difficult, but relative values of g, or
differences between its values at different places, can be determined with
very high precision by means of pendulum observations.
If t and t' be the times of oscillation of a certain pendulum at two
stations, the corresponding values g and g' of gravity are connected by the
relation g'/g = (t/t') 2.
This enables gravity at any station to be determined in terms of gravity
at a base station. For accurate work corrections have to be applied to the
observed times of swing to allow for departures of temperature and pressure
from their standard values, also for chronometer-rate and flexure of the
pendulum-stand. When all the known corrections are carefully made a
very high degree of accuracy is obtainable. For instance, taking 981 -200
as the value of g at Kew Observatory, this being the value accepted for
the purposes of the Trigonometrical Survey of India, the last two comparisons
instituted between Greenwich and Kew, the one made by the United States
Coast and Geodetic Survey, the other by the Trigonometrical Survey of
India—using two different sets of half-second pendulums, gave for g at
Greenwich the respective values 981 -188 and 981 • 186.
Pendulum and other geodetic observations have led to a theory of
isostasy which has received strong support of late years, especially in the
United States. According to this theory if we start at about 100 Km. below
sea level we find between there and the free surface an approximately
uniform quantity of matter irrespective of whether the free surface is
mountainous or not. A lesser density under lofty mountains and a higher
density under deep seas act as compensations.
While the mass of a body is independent of its position, its weight,
i.e. the gravitational attraction exerted on it, varies with g and so increases
as we pass from the equator towards either pole. Denoting by gf the value
of g in lat. cj> we obtain from the formula
£9U = 983-19,
£0 = 978-00.
In other words, gravity at the poles exceeds gravity at the equator by 1 part
in 189.
Green Flash.—The " green flash " is a phenomenon which is observed
fairly often at sunset, more rarely (because seldom looked for) at sunrise.
At sunset the last glimpse of the sun which is seen is a brilliant green. The
colour lasts but two or three seconds. The explanation is that the rays from
the sun are refracted in their passage through the atmosphere, the blue and
green more than the yellow and red so that when the sun is so low that no
red light from it can reach the observer, green light from the upper limb
can. The blue light is generally absorbed or diffracted by the atmosphere.
The phenomenon requires a clear atmosphere ; whether unusual refractive
power is also necessary has not been settled.
Gregale.—A strong NE. wind occurring chiefly in the cool season in the
south-central Mediterranean. The name has been applied to strong NE.
winds in other localities of the Mediterranean ; it is used, for example, on the
south coast of France (gr6gal) and in the Tyrrhenian Sea (grecale). In the
neighbourhood of Malta gregale occurs most often, when pressure is high over
central Europe and the Balkans and relatively low to the south over Libya ;
with this type there is little or no rain, visibility is good apart from spray and
temperature is not much below normal. Gregale also sets in with the passage
of the cold front of a depression moving south-eastwards across the Ionian
GROUND FROST
104
Sea or Greece; with this type heavy showers of rain
occur, with low
temperature. A third type of gregale at Malta is due tomay
depre
to the south, the atmosphere is usually thick, rain may or may notssions passing
fall. Gregale
also blows at times with small local depressions.
Ground Frost.—As injury to the tissues of growi plants is
not caused
until the temperature has fallen appreciably belowngthe
freezi
ng
point of
water (32° F.) a " ground frost " is regarded as having occur
red
when
the
thermometer on the grass has fallen to 30° F. or below. If the
therm
omet
er
is read to tenths of a degree the limit is 30-4° F.
Ground, State of.—Observations of the state of
ground are made
regularly at a number of stations in this-country intheconne
operations of aircraft and of agriculture. The code provides xion with the
for 10 different
conditions of the ground in the vicinity of the station, such
flooded, frozen, partly covered with snow or hail, covered with as dry, wet,
frost, with thawing snow, or covered with snow less or more than ice or glazed
and frozen or not frozen. The code was approved for gener 6 inches deep
International Conference of Directors which met at Warsaw al use by the
1935. A modified code is used at CROP-WEATHER stations. in September,
Gulf Stream.—The Gulf Stream is one of the stron
and most constant
ocean currents. Originating in the eastern area of gest
the
Gulf of Mexico it
flows through the Straits of Florida and up the eastern coast
s of the United
States. The Gulf Stream does not flow inshore, but generally speak
ing follows
the edge of the continental shelf. It is continued by a weak
er
and
broader
current, often called the Gulf Stream, but more accurately the
North
Atlan
tic
Drift, right across the Atlantic Ocean, leaving the coast of Amer
ica
in
abou
t
latitude 40° N. and reaching the British Isles in about latitude
50°
N.
Like
all ocean currents it is subject to variability and even revers
e sets. Recent
research shows that up the American coast the GulftoStrea
m
spring and summer, when it flows with a mean strength of 30 is strongest in
The transatlantic current has a mean strength of 3-5 miles permiles per day.
autumn the Bahama current flowing to the eastward of the day. In the
the Gulf Stream in about latitude 30° N., at other seasons itBahamas joins
interrupted. It is popularly supposed that the temperate is partial and
climate of the
British Isles is due to the warmth conveyed by the Gulf Strea
real state of affairs is that our climate is due to the preva m, but the
and south-westerly winds, which also cause the extension of theiling westerly
across the Atlantic. For a discussion of the effects of the GulfGulf Stream
the weather of western Europe see London, Geophysical Memo Stream on
irs, No. 34,
1926, and London, Meteorological Magazine, 63, 1928, p. 61.
Gust.—The word was used originally for any transient blast
of wind,
but is now limited to the comparatively rapid fluctu
ations
in
the
strength
of the wind, which are characteristic of winds near the surfac
e
of
the
earth
and are mainly due to the turbulence or eddy motion arising from
the
frictio
n
offered by the ground to the flow of the current of air.
An investigation regarding the nature of gusts, as indicated
by a tubeanemograph, was carried out by the Advisory Committee
for
Aeron
autics
and the results are contained in several reports on wind struc
ture
publi
shed
in the annual reports of the Committee. The number and
exten
t
of
the
fluctuations are very irregular ; the range between gusts and lulls
is
depen
dent
on the mean velocity of the wind and the exposure of
anemometer. Other
factors also play their part, and winds from certaithe
n
quart
ers tend to be
more gusty than those from others. Expressing the fluctuations
as a per­
centage of the mean velocity the following results were obtai
ned
for
anemometers (Report of the Advisory Committee for Aeronautics various
, 1909-10,
pp. 103-4).
HABOOB
105
Anemometer
Range of fluctuation as a
percentage of the mean
velocity
Southport (Marshside)
Stilly (St. Mary's)
Shoeburyness, ENE. wind ..
„
W. wind
Holyhead (Salt Island)
Falmouth (Pendennis), S. wind
,,
,,
W. wind
Aberdeen
Alnwick
Kew ..
30
50
30
80
50
25
50
100
80
100
In this table a fluctuation of 100 per cent, means that a wind with a
mean velocity of, say, 30 mi./hr. fluctuates over a range of 30 mi./hr.,
between about 15 mi./hr. and 45 mi./hr., in consequence of the gustiness.
Gusts are to be distinguished from squalls. A squall is a blast of wind,
occurring suddenly, lasting for some minutes, and dying away as suddenly.
Gusts are the result of mechanical interference with the steady flow of air,
whereas squalls are attributable to meteorological causes.
The strongest gusts recorded on anemometers of the Meteorological Office
in recent years are :—
1922 ..
1925 ..
1927 ..
1928
1929
1932
1935
1936
1937
..
..
..
..
..
..
1938 ..
1941 ..
1943 ..
1945 ..
^10
..
Pendennis
Lerwick . .
Southport
Paisley . .
..< Tiree
1 Dunfanaghy
Plymouth
Pendennis
../ Stilly
Valentia . .
../ Abbotsinch
Bell Rock
Point of Ayre
Holyhead
r Bidston . .
. J Mildenhall
Stomoway
Lizard
Manchester (Rin •way)
. J Prestwick
Spurn Head
St. Ann's Head
Lerwick . .
Stornoway
Tiree
Lizard
Scilly
MildenhalV
Cardington
Durham . .
. .-.
St. Ann's Head
m./sec.
46-0
42-5
42-9
46-5
48-3
48-7
42-9
46-0
49-1
42-9
41-1
45-2
40-2
47-8
45-2
40-7
43-8
42-0
40-7
40-7
40-7
50-5
40-2
44-3
40-2
42-0
44-7
43-8
41-6
42-5
41-6
mi./hr.
103
95
96
104
108
109
96
103
111
96
92
101
90
107
101
91
98
94
91
91
91
113
90
99
90
94
100
98
93
95
93
A gust at Quilty in 1920 was reported as greater than 112 mi./hr., at
which point the record left the chart, but this is now regarded as of doubtful
authenticity.
Haar.—A local name in eastern Scotland and parts of eastern England for
a kind of wet sea fog which at times invades coastal districts. Haars occur
most frequently in summer months.
Haboob.—The name is derived from the Arabic " habb " meaning to
blow and is used generally to imply the passage of a dense mass of whirling
(90565)
D*
HAIL
106
dust which is usually accompanied by a sudden increase in the strength of
the wind with a change of direction, by a sharp fall of temperature and by
very low visibility on account of the dust raised, while often it is followed
by heavy rain and sometimes by thunderstorms. Haboobs occur in north
and north-eastern Sudan and are most frequent near Khartoum. At Khartoum
they are chiefly experienced from May to September, though they may occur
at any time ; they have a pronounced diurnal frequency being rare between
4 a.m. and 2 p.m. and most common in the afternoon and evening. Most
haboobs appear to be due to a current of relatively cold air undercutting
warm air ; occasionally the cold air is due to the passage of a shallow depression
across northern Sudan. See L. J. Sutton, London, Quart. J.R. met. Soc., 57,
1931, pp. 143-61.
Hail.—The term properly denotes the hard pellets of ice of various
shapes and sizes and more or less transparent, which fall from cumulonimbus
clouds and are often associated with thunderstorms.
Hailstones may attain a great size, stones as large as grape-fruit have
been observed and the recorded weights range up to a kilogram (over 2 Ibs.).
An important characteristic of a cumulonimbus cloud is the rapid ascent
into it of a moist current of air, which is quickly cooled by reason of the
reduced pressure which it encounters aloft. As the process continues cloud
particles, and finally rain-drops, are formed, but if the ascensional velocity
of the air exceeds 8 m./sec. all the condensed water is retained in the cloud
(see RAIN-DROPS). At a level not usually exceeding 4 Km. in this country
the temperature has already fallen below freezing point. There is much
evidence for believing that, at least in many cases, ice crystals are not imme­
diately formed as soon as the temperature falls below freezing point, but
that water-drops are still produced. These are in the supercooled condition,
and they are carried upwards into the higher part of the cloud. Near the
top of this cloud, however, ice crystals will appear and these will grow into
pellets of small hail by a process of condensation direct from vapour to ice.
When the weight of a pellet is sufficient to overcome the resistance of the
upward air current it will commence to fall.
During the downward journey the pellet will fall through upward-moving
air containing supercooled water-drops and its size will increase by the
deposition upon it of the water so encountered, provided that freezing occurs
when the water unites with the hailstone. The growth of the hailstone therefore
depends upon (a) the liquid water content of the air, (b) its upward velocity
and (c) the heat exchanges between the hailstone, the air and the supercooled
water. Schumann has shown that the growth of spherical hailstones to
diameters of the order of 8 cm. will occur on reasonable assumptions as to
the magnitude of these quantities.
The international symbol for hail is A- Two other varieties, soft hail
(Reifgraupeln -^f) and small hail (Frostgraupeln A) are recognised. See
Fig. 30 p. 212.
Soft hail consists of white opaque grains 2 to 5 mm. in diameter having a
snow-like structure; it occurs mainly at temperatures about the freezing
point and mostly on land, often before or together with ordinary snow.
Small hail consists of semitransparent, round (occasionally conical)
grains of frozen water, 2 to 5 mm. in diameter. They generally have a nucleus
of soft hail, covered by a very thin layer of clear ice, and occur in association
with rain rather than with snow. They generally fall from cumulonimbus
clouds.
Halo Phenomena.—The term halo, which might be applied to any circle
of light round a luminous body, is restricted by meteorologists to a circle
produced by refraction through ice crystals; in contrast to coronae which
are produced by diffraction. All the optical phenomena produced by re­
flection and refraction of light by ice crystals are sometimes grouped together
as halo phenomena.
107
HARMONIC ANALYSIS
The most common halo is a luminous ring of 22° radius surrounding the
sun or moon, the space within the ring appearing less bright than that just
outside. The ring, if faint, is white; if more strongly developed the inner
edge is a pure red, outside which yellow may be detected. The halo of 22°
is very common. In England it can be seen by an assiduous observer about
one day in three.*
The angle of 22° is the angle of minimum deviation for light passing
through a prism of ice (index of refraction 1-31) with faces inclined at 60°.
Thus the occurrence of the halo of 22° radius indicates the presence of ice
crystals with faces inclined at 60°. Alternate faces of a hexagonal prism
are inclined at this angle, and as hexagonal prisms are frequently found
amongst ice crystals the halo is probably due to the refraction of light through
such prisms.
A halo of 46° is to be seen occasionally, though seldom complete. This
halo requires crystals with faces at right angles.
The halo of 22° is sometimes within a circumscribed nearly elliptical
halo the points of contact being at the highest and lowest points. The
complete circumscribed halo is only seen when the elevation of the sun is
40° or more. With lower elevations separate tangent arcs are seen. These
phenomena are explained by the presence of prismatic ice crystals floating
with their axes horizontal.
Another group of phenomena requires prismatic crystals with their
axes vertical. In this group are parhelia, or MOCK SUNS (patches of light
at the same elevation as the sun) and the circumzenithal arc (a horizontal
circle rather more than 46° above the sun).
In weather lore halos are often spoken of as presaging storms. In modern
meteorology the cirrostratus cloud in which halos are likely to be observed
is regarded as a feature of a WARM FRONT. Halos are too common, however,
to be good signs of exceptional weather.
Harmattan.—A dry wind blowing from a north-east or sometimes easterly
direction over north-west Africa. Its average southern limit is about 5° N.
latitude in January and 18° N. in July. Beyond its surface limit it continues
southwards as an upper current above the south-west monsoon. Being both
dry and relatively cool, it forms a welcome relief from the steady damp
heat of the tropics, and from its health-giving powers it is known locally as
THE DOCTOR, in spite of the fact that it carries with it from the desert great
quantities of impalpable dust, which penetrates into houses by every crack.
This dust is often carried in sufficient quantity to form a thick haze, which
impedes navigation on the rivers.
Harmonic Analysis.—There are many meteorological phenomena which
recur with some approach to regularity day by day. If the changes of such
a variable as temperature are represented by a curve, then the portions
corresponding to successive days bear a strong likeness to one another. If
for the actual record for each day the record for the average day were
substituted, the variation for a long period would be represented by a curve
in which the part corresponding with each day was like its fellows. The
simplest curve possessing this property of continuous repetition is a curve
of sines. As an example the variation of temperature at Kew Observatory,
Richmond, in July, may be cited. The sequence of change throughout the
average day is shown in the lower part of Fig. 20. The representative curve
is not unlike a curve of sines, but it is not quite symmetrical. The rise which
commences at sunrise and lasts until after 15 h. is more steady than the drop
which is rapid in the evening and slow after midnight. A good approximation
to the temperature on the average day is given, however, by the expression
289-9 + 3-7 sin (15* + 224£°) where t is the time in hours reckoned from
midnight. It will be seen that the lowest value is reached at the time given
* At the Radcliffe Observatory, Oxford, the average number of days per year on
which solar halos were observed was 152, while the average number of nights per year
with lunar halo was 38.
(00565)
D*2
HARMONIC ANALYSIS
108
by 15* + 224£° = 270°, i.e. at 3h. 6m. and the maximum comes 12 hours
later at 15h. 6m. The substitution of a sine curve for the curve based on
the observations would make the minimum too early by an hour, but would
not affect the maximum so much.
The curve showing the diurnal variation of temperature in March (in
the upper part of the figure) is not so near to a sine curve as that for July.
The rise in temperature from minimum to maximum takes little more than
six hours. The best sine curve for representing the variation is given by
the formula
6 = 278-74 + 2-47 sin (15* + 222°)
and is shown by the broken line in the figure. It will be seen that the
agreement is by no means close. To obtain a more accurate expression for
the temperature, an additional sine term with a period of 12 hours may be
introduced. The best formula containing such a term is
6 = 278-74 -f 2-47 sin (15* + 222°) + 0-63 sin (30* + 39°)
OBSERVATORY, RICHMOND, SURREY -«^P
e^»-KEW
-NORMAL DIURMAL VARIATION IN TEMPERATURE IN MARCH
a
zre ——
f^a
1
*•
"rom Hourly Mean *
^^~^^
\~^
f"
Z80
282
JNSET
280
/
278
^--^
276
1
1
1
1
292
Jf
i
,
,
1
218
./
SUNRISE
^~~-^_^___^
^^^^
,
—1
,
•
1
276
.
.
III,
i
264
FIRST TERM OF HAR IONIC SERIES---FIRST TWO
282
28Z
^^^"^
280
sj?
..-^
-?T^~^—-~^
. i i i i
i i i i i
278
Z!b
,
/
280
X<<^
^N~"
278
276
.
ill,
i
,
1
i
,
NORMAL DIURNAL VARIATION IN TEMPERATURE IN JULY.
294
2<J4
292
zyo
288
SUNRISE
^•V^^
/
286
1
GM.T
1
1
,
i
24-6
/
1
1
8
/
M<
~^V
OPI^^-^"
s
\
292
\ SUNSET
290
\^
^^
1
1
10
288
28b
1
i
12
i
14
1
i
16
i
I
18
1
20
1
1
2.2
^
Z4
FIG. 20.
The new term 0 • 63 sin (30* + 39°) is positive in the early morning and in the
early afternoon, so that it delays the drop to the minimum and makes the
maximum earlier. In the upper part of Fig. 20, the continuous curve which
109
HARMONIC ANALYSIS
corresponds with the proposed formula crosses the simple sine curve at
intervals of six hours. The resemblance to the curve based on the observations
is greatly improved. A closer resemblance would be obtained if additional
terms
0-08 sin (45t + 330°) +0-12 sin (60/+ 190°)
were included in the formula.
The harmonic representation of a diurnal inequality may be expressed
in either of the alternative forms—
ax cos (15" X t) + a 2 cos (30° X /) + a, cos (45° X t)
+ a4 cos (60° X t) + . . .
+ &! sin (15° X t) + 6 2 sin (30° X t) + b3 sin (45° X /)
+ &4 sin (60° X t) + . . . ,
P1 sin (15° X t + AJ + P2 sin (30° x t + A z) +
P3 sin (45° X t + A 3) + Pt sin (60° X t + A t) + . . . .
where t denotes the time in hours counting from some fixed hour, usually
midnight. The latter is the form which has been adopted in the previous
part of this note, as it best exhibits the physical significance of the results,
but the first form is that employed for the actual numerical calculation of
the harmonic coefficients. We first calculate the a, b coefficients and then
derive the P, A coefficients from the relations
tan A n = an/bn ; Pn = an/sm A n = bn/cos A n ;
Pn* = 0** + &„*
where n may be 1, 2, 3, 4, etc.
Details of the methods adopted for practical computation of the coefficients
will be found in the " Computer's Handbook."
The process of finding the trigonometric series to give the best representation
of a periodic function is known as harmonic analysis. The reverse process,
determining the value of the function at any time when the components are
known, is harmonic synthesis. Both processes can be carried out by suitable
machines, and also by arithmetical computation from given data. The latter
process is the more usual except in the case of the prediction of tides.
In any term P sin (nt -f- A ) the coefficient P which determines the range
is called the amplitude and nt + A is called the phase angle, A being the
phase angle for midnight. It may be mentioned that the alternative form
P cos n (I — t0) where t0 is the time of the maximum, has certain advantages ;
it was adopted by General Strachey for the discussion of harmonic analysis
of temperature in the British Isles.
By comparison of the amplitudes and phase angles for different places
and different seasons, climates may be classified. For example, the amplitude
of the whole-day term for temperature in July at Falmouth is 2-1° A., and
the phase angle for local apparent midnight is 250°. In comparison with
Kew, the amplitude is small and the maximum occurs early. This difference
in phase is typical of the difference in conditions on the coast and inland. It
may be stated, however, that as regards temperature, harmonic analysis
has not yielded information which cannot be obtained more readily from the
curves showing the daily variation. With pressure more important results
have been discovered.
For temperature, the first or all-day term in the expansion in trigonometric
series is by far the most important. With pressure the second term, is
comparable in size with the first, and at most stations there are two maxima
and two minima in the course of 24 hours. The first term is found to depend
on the situation of the station, whether near the coast or inland, in a valley
or on a mountain-side, whereas the second or twelve-hour term depends
principally on the latitude. The daily changes represented by the first term
are clearly understood, they are the effects of local heating of the air.
HAZE
110
The daily variation of pressure at Cairo in Tulv is represented graphically
by Fig. 21.
-I Mb'
FIG. 21.—Daily variation of the Barometer at Cairo (Abbassia Observatory) in July.
The departure of the pressure from the mean for the day is given in
millibars by the expression
•92 sin (15* + 17°) + -66 sin (30* -f- 140°)
+ • 12 sin (45* + 348°) + -05 sin (60* + 250°).
The first term represents an oscillation with the maximum and minimum
at about 5h. and 17h. respectively. It indicates that as the air is warmed
in the daytime it expands and overflows from the Nile valley over the
surrounding high ground and over the neighbouring seas.
The second term represents an oscillation with maxima at lOh. 20m. and
22h. 20m. These hours are almost the same all over the globe. The amplitude
depends on the latitude and to a certain extent on the time of the year. The
mean value for the year at Cairo is 0-8 mb. It is about 1-3 mb. at the
equator, 0-5 in latitude 45°, 0-35 in London and 0-1 mb. in latitude 60°.
The third term is interesting as it changes its phase by 180° at the
equinoxes. The first maximum occurs at 2h. in summer, the first minimum
at the same hour in winter.
It has been mentioned that the all-day term depends largely on local
conditions. An interesting contrast is offered by the British observatories.
At Richmond, Surrey, the amplitude of this term in July is about 0-3 mb.,
and it has about the same value at Valentia, but the phases are opposite ;
at Richmond the maximum occurs at 5h., whereas at Valentia it is the
minimum which occurs in the morning (at 7h.).
Harmonic analysis may be extended to the investigation of changes which
are caused by forces having different periods. The classical instance is that
of the tides. The tides being caused by the attraction of the sun and the
moon show as periods the solar and also the lunar day. The process by
which the heights and the times of tides are foretold in practice depends on
harmonic analysis and synthesis.
Haze.—See FOG.
Heat.—The name used for the immediate cause of the sensation of warmth,
a primary sensation which is easily recognised and needs no explanation.
As used in relation to the weather, heat and cold are familiar words for
opposite extremes of temperature of the air. What the American writers
call a heat wave is a spell of hot weather in which the maximum temperatures
reach 90° or 100° F. (above 305°A.), and a cold wave is a spell of the opposite
character during which temperatures in the neighbourhood of the Fahrenheit
zero, or 32 degrees of frost may be experienced. In continental climates,
during the passage of severe cyclonic depressions, the transitions from heat
to cold are sometimes extremely abrupt and far-reaching; a difference of
temperature of 50° F. in a few hours is not unknown. We have visitations
of similar character in this country, but they are less intense. A few days in
succession with a temperature over 80° F. would suffice for a heat wave,
and a few days with 10° of frost would certainly be called a cold wave. One
of the most noticeable features of our climate is the succession of cold spells
which interrupt the genial weather of late spring and early summer. They
are not very intense, but a drop in the mean temperature of the day from
55° F. to 45° F., which roughly defines them, produces a very distinct
impression.
Ill
HEAT
As used in connexion with the study of the atmosphere, heat has another
sense which must not be overlooked. It denotes the physical quantity,
the reception of which makes things warmer, and its departure makes them
colder. If you wish to make water hot, you supply heat to it from a. fire
or a gas-burner or, in modern days, by an electric heater, a very convenient
contrivance for getting heat exactly where you want it. On the other hand,
if you want water to become cooler, you leave it where its heat can escape,
by CONDUCTION, aided by CONVECTION or by RADIATION. You can also warm
water by adding some hot water to it, or cool it by adding cold water to it.
Either process suggests the idea of having the same quantity of heat to deal
with altogether, but distributing it, or diluting it, by mixing.
The idea of having a definite quantity of heat to deal with, and passing
it from one body to another is so easily appreciated and so generally applicable,
that the older philosophers used to talk confidently of heat as a substance
which they called Caloric, and which might be transferred from one body to
another without losing its identity. They measured heat, as we do still, by
noting by how much it would raise the temperature of a measured quantity of
water. For students of physics the unit of heat is still a gram-calorie, the heat
which will raise a gram of water through one degree centigrade. To raise m
grams from tj° C. to tz° C., m (t2 — tj gram-calories are required. The amount
can be recovered, if none has been lost meanwhile, by cooling the water. If
we wish to be very precise, a small correction is required on account of
the variation in what is called the capacity for heat of water at different
temperatures, but that need not detain us.
For students of engineering, the unit called the British Thermal Unit,
is a pound-Fahrenheit unit instead of the gram-centigrade unit, and the
heat required to raise m pounds of water from tj° F. to tz° F.is m (tz — tj) B.Th.U.
It is in many ways a misfortune that students of physics and engineering
do not use the same unit. It is no doubt a good mental exercise to learn to
use either indiscriminately without confusion, but it takes time.
From measurements of heat we get the idea that with different substances
the same change of temperature requires different quantities of heat; the
substances have different capacities for heat. We define capacity for heat as the
heat required to raise a unit of the substance (1 gram or 1 Ib.) through 1 degree.
It is a remarkable fact that of all common substances water has the
greatest capacity for heat. It takes one unit to raise the temperature of a
unit mass of water one degree, it take less than a unit, sometimes only a small
fraction of a unit, to raise the temperature of the same amount of another
substance through one degree. We give the name specific heat to the ratio
of the capacity for heat of any substance to the capacity for heat of water.
Numerically, specific heat is the same as the capacity for heat in calories
per gram.
The specific heat of water is 1, the specific heat of any other common
substance is less than 1. The specific heat of copper is only 1/11. So the
heat which will raise the temperature of a pound of copper 1° will only raise
the temperature of a pound of water 1/11°, or the heat which will raise the
temperature of a mass of water 1° will raise the temperature of the same
mass of copper 11°.
This peculiar property of water makes it very useful for storing heat
and carrying it about. From that point of view it is the best of all substances
for cooling the condenser of an engine, for distributing heat at a moderate
temperature in a circulating system, and for many other economic purposes.
In meteorology its influence is very wide. Large masses of water, of
which the ocean is a magnificent example, are huge storehouses which take
up immense quantities of heat from the air when it is warm and give it out
again when the air is cold, with very little change in its own temperature,
so that a large lake, and still more the ocean, has a great influence in reducing
the extremes of temperature of summer and winter, and of day and night,
in the countries which border it.
HEAT
112
There is another remarkable storage of heat in which water takes a
predominant share that is dealt with in physical science under the name of
latent heat.
Water at 288°A. (59° F.) cannot be evaporated into water vapour unless
every gram of it is supplied with 589 calories of heat, which produce no
effect at all upon the temperature. The water is at 288°A. to start with,
and the water vapour is at exactly the same temperature and yet 589 calories
of heat have gone. They are latent in the water vapour but produce no effect
on the thermometer. You can get them back again easily enough if you
condense the vapour back again into water, but you must manage somehow
to take away the heat while the condensation is taking place. The separation
of the " waters that are above the firmament from the waters that are below
the firmament," or in modern language, the evaporation of water from the
sea or a lake or the wet earth and its condensation in the form of clouds and
rain, implies the transference of enormous quantities of heat from the surface
to the upper air, the dynamical effect of which belongs to another chapter
of the romantic story of heat which deserves more than the few words which
we can afford for it. Readers can find an interesting account in Tyndall's
" Heat, a Mode of Motion."
The idea of heat as an indestructible substance, caloric, which could be
transferred from one body to another without loss, became untenable when
it was found that when air was allowed to expand in a cylinder it cooled
spontaneously to an extent that corresponded exactly, so far as could be
ascertained with the means then available, with the amount of mechanical
work that the cylinder was allowed to do. It was the last step in the process
of reasoning by which men had come to the conclusion that, when mechanical
work was devoted to churning water or some other frictional process, heat
was actually produced, not brought from some other substance but created
by the frictional process.
It took many years for men to reconcile themselves to so novel an idea,
and a good deal of ingenuity was devoted to trying to evade it, but it has
now become the foundation stone of physical science. Heat is not an
unalterable indestructible substance but a form of energy. It can do
mechanical work in a steam-engine or a gas-engine or an oil-engine, but for
every foot-pound* of work that is done a corresponding amount of heat
must disappear, and in place of it a corresponding amount of some other
form of energy is produced. A good deal of heat, besides, may be wasted in
the process so far as practical purposes are concerned. In a steam-engine,
of the whole amount of heat used, only one-tenth may be transformed, the
rest wasted, as we have said ; but it is still there raising the temperature
of the water of the condenser or performing some other unproductive but
necessary duty.
There is, therefore, a numerical equivalent between heat and other forms
of energy.
We give the relation :—
1 B.Th.U. is equivalent to 777 foot-pounds of energy.
1 gram-calorie = 42,640 gram-centimetres.
= 41,830,000 ergs.
We have led up to this statement in order to point out how extraordinarily
powerful heat can be in producing mechanical energy.
If, in the operations of nature, one single cubic metre of air gets its
temperature reduced by 1° C. in such a way that the heat is converted into
work by being made to move air, the equivalent of energy would be a cubic
metre of air moving with a velocity of nearly 45 m./sec. (101 mi./hr.).
* A foot-pound of work is the work done in lifting one pound through a distance of one foot.
A gram-centimetre is the work done lifting one gram through one centimetre.
An erg is the absolute unit of work on the C.G.S. system ; 1 gram-centimetre = 981 ergs.
113
HIGH
So familiar have we become with heat as a form of energy that we measure
the heat of sunlight in joules* and the intensity of sunshine in watts per
square centimetre, i.e. the number of joules falling on one square centimetre
per second.
The specific heat of air.—The foregoing statement is necessary to lead up
to a matter of fundamental importance in the physics of the atmosphere,
namely the heat that is required or used to alter the temperature of air in
the processes of weather ; in technical language this is the capacity for heat
of air or the specific heat of air.
We have explained that when air is allowed to do work on its environment,
in expanding, heat disappears, or more strictly is transformed. So the amount
of heat required to warm air through a certain number of degrees depends
upon how much expansion is allowed during the process. The most economical
way of warming air from the thermal point of view is to prevent its expanding
at all; it then has "constant volume" and its specific heat is 0-1715
calories per gram per degree at 273°A. It is remarkable, but true, that if
you have a bottle full of air, it will take more heat to raise the temperature
of each gram of it by a degree if you take the stopper out while the warming
is going on, than if you keep it tight. The difference between warming a bottle
of air with the stopper in and with it out, simple as it may seem, has got in
it the whole principle of heat as a form of energy. The effect of leaving the
stopper out is that the pressure of the air inside the bottle is the atmospheric
pressure for the time being and is therefore practically constant throughout
the brief operation. The specific heat of air at constant pressure is 0-2417
gram-calories or 1-010 joules per gram per degree Centigrade. The specific
heat of air at constant volume is 0-72 joules per gram per degree Centigrade.
The difference of the two represents the heat equivalent of the work used in
expanding unit mass of the gas against atmospheric pressure.
Helm Wind.—A strong, often violent north-easterly wind blowing down
the western slope of the Crossfell Range in Westmorland and Cumberland,
which on account of its origin is in general decidedly cold. It may occur
at all seasons of the year provided that the general direction of the surface
wind is between NNE. and E. As a rule, therefore, it is most common in late
winter and spring. When the " helm wind " is blowing, a heavy bank of
cloud (the " Helm ") rests along or just above the Crossfell Range, and at a
distance of one to four miles from the foot of the Fell, a slender, nearly
stationary roll of whirling cloud (the " Helm Bar ") appears in mid-air and
parallel with the " helm " cloud. The cold wind blows strongly down the
steep fell sides until it comes nearly under the Bar, when it suddenly ceases ;
approaching this point it is characteristically very gusty and may cause
structural damage. To the west of this calm at the surface, a slight westerly
wind may be experienced for a short distance. In dry weather the space
between the Helm and the Bar is usually quite clear although the rest of the
sky may be cloudy, and the strong wind has a marked desiccating effect ;
in rainy weather, very little rain falls in the region where the strong wind is
experienced. When the uninterrupted depth of the north-easterly surface
current up to the summit level of the clouds is greater than about 6,000 ft.,
the bar with its underlying reverse current at ground level is no longer found.
North-easterly wind with cloud is not uncommonly referred to as " helm wind "
elsewhere, e.g. in the Lake District but the full development is confined to the
strip of country east of the River Eden, particularly near Crosfell itself.
High.—Sometimes used to indicate an area of high barometric pressure
for which the technical term ANTICYCLONE was coined by Sir Francis Galton.
The central region of an anticyclone on a weather chart is frequently indicated
by the word " high."
* A joule is a more convenient unit than the small unit, the erg ; one joule=ten million
ergs (10 7 ergs) and one calorie=4-18 joules.
The watt is a unit of power, that is, rate of doing work: a power of one watt does one
joule of work per second.
HOAR-FROST
114
Hoar-Frost.—Thin ice crystals in the form of scales, needles, feathers or
fans deposited on surfaces cooled by radiation. The deposit is frequently
composed in part of drops of dew frozen after deposition and in part of ice
formed directly from water vapour at a temperature below the freezing point.
Horizon.—The " sensible or visible horizon " which is visible from a
ship at sea, the line where sea and sky apparently join, is a circle surrounding
the observer a little below the plane of the horizon in consequence of the
level of the earth's surface being curved and not flat. The depth of the
" sensible horizon " below the " rational horizon " or horizontal plane is
approximately the same as the elevation of the point from which the
" sensible horizon " is viewed. Apart from any influence of the atmosphere
the distance of the visible horizon for an elevation of 100 feet (30 metres)
is about 12 miles. The actual distance is about 2 miles greater on account
of refraction. It varies as the square root of the height, so that it would
require a height of 400 feet to give a horizon 24 miles off. A level canopy
of clouds 10,000 feet high is visible from a point on the earth's surface for
a distance of about 125 miles, or the visible canopy has a width of 250 miles.
The Visible Horizon, or Distance of Visibilik| for objects of given height
1000
10.0.00
/
900
9.000
/
800
700 «>
£
600 c
r/
500
<0|
400
i;
300
/
200
too
-^
JT , /
/7
o 7.000
/
/
r
/
'^•^ ^
/
^
/
_s
/
E
»
i
6:000
5
5.000
^ 4000
oT
/
3.000
2.000
Cnrvf T reft r": to I ,jt
H
8000
3-
/
«Ki ghr
H.
•
nrl H, t%M < c.n\f
••
»»
1,000
150 Miles
120
110
100
9O
60
70
60
50
40
30
20
ic
FIG. 22.—Diagram showing the relation between the height of an
observation point in feet and the distance of the visible horizon in miles
(neglecting refraction), or the height in feet of a cloud or other distant
object and the distance in miles at which it is visible on the horizon.
MHes. C;
Horse Latitudes.—The belts of calms, light winds and fine, clear weather
between the TRADE WIND belts and the prevailing westerly winds of higher
latitudes. The belts move north and south after the sun in a similar way to
the DOLDRUMS. The name arose from the old practice of throwing overboard
horses which were being transported to America or the West Indies when the
ship's passage was unduly prolonged.
Humidity.—The condition of the atmosphere in respect to water vapour.
Air may be described as " humid " when it has a high moisture content, but
when the word " humidity " is employed without a qualifying adjective the
RELATIVE HUMIDITY is usually meant. See AQUEOUS VAPOUR.
Humidity-mixing-ratio.—The relative proportions by weight of water
vapour and dry air, in a given specimen of damp air. It is usually denoted by
the symbol x. Thus x is the number of grams of water vapour mixed with
1 gram of dry air. If e is the vapour pressure, p the total pressure, and
e the ratio of the densities of water vapour and dry air, (e = 0-622)
ee
* = I——
p-e
The humidity-mixing-ratio, as usually defined, is, as stated above, the number
of grams of water vapour mixed with one gram of dry air, but in practice,
115
HYGROMETii
particularly in the use of the tephigram, it is more convenient to define it as
the number of grams of water vapour mixed with one kilogram of dry air.
This avoids the use of many places of decimals.
The term " Mixing ratio ", without the word " Humidity ", was introduced
by W. von Bezold in one of his earlier papers " On the thermo-dynamics of
the atmosphere ", of which a translation is in the " Mechanics of the earth's
atmosphere " by C. Abbe (Smithson. misc. Coll., Washington, Vol. XXXIV,
1893), but the first reference to the term " Humidity-mixing ratio " is believed
to be by C. W. B. Normand in " Wet-bulb temperatures and the thermo­
dynamics of the air " (Indian met. Mem., Calcutta, Vol. XXIII, No. 1, 1921).
Hurricane.—A name (of Spanish or Portuguese origin) given primarily
to the violent wind storms of the West Indies and Gulf of Mexico, which are
CYCLONES with diameters of from 50 to 1,000 miles, wherein the winds reach
hurricane force near a central calm space; the whole system advances in a
straight or curved track. Similar tropical revolving storms off the coasts
of Queensland are also known as hurricanes.
In the BEAUFORT SCALE OF WIND FORCE the name hurricane is given to
a wind of force 12, and its velocity equivalent is put at a mean velocity
exceeding 34 m./sec. or 75 mi./hr., but from the remarks under GUSTINESS it
will be understood that at ordinary exposures a wind with a mean velocity
of 75 mi./hr. will include gusts of considerably higher velocity, reaching
100 mi./hr. or more. Hurricane winds in this sense are occasionally experienced
in this country.
Hydrometer.—An instrument for measuring the density of liquids. In
marine meteorology hydrometers are used for determining the density of
sea water.
Hydrometeor.—A generic term for weather phenomena such as rain,
cloud, fog, etc., which mostly depend upon modifications in the condition of
the aqueous vapour in the atmosphere.
Hyetograph.—A pattern of self-recording RAIN-GAUGE in which the
recording pen is actuated by a series of stops attached to a vertical float rod.
Hygrodeik.—A dry- and wet-bulb PSYCHROMETER mounted on a frame,
graduated and furnished with an index for determining quickly the relative
humidity, dew-point, etc., by a graphical method.
Hygrograph.—An instrument for recording continuously the humidity
of the atmosphere. See HYGROMETER.
Hygrometer.—An instrument for determining the humidity of the air.
The dew-point hygrometer, developed by Daniell, Regnault and others, aims
at determining the dew-point by artificially cooling, until dew is deposited,
a polished surface whose temperature can be measured. Since dew-point
hygrometers depend only on the measurements of a temperature and involve
no assumption as to the properties of the materials or the laws of evaporation
from a wet surface, they may be classed as " absolute " instruments. To the
same class belong chemical hygrometers in which the quantity of moisture in
a given mass of air is determined by direct weighing. Unfortunately, neither
dew-point nor chemical hygrometers are suitable forroutine use in meteorology,
and it is therefore necessary to use methods depending on (a) the change of
length of hairs with varying relative humidity, or (b) the lowering of
temperature due to evaporation.
In the hair hygrometer, of which Lambrecht's polymeter and Richard's
hair hygrograph are applications, use is made of the fact that human hair
expands with increasing relative humidity. Hair hygrometers are incapable
of high precision, but they possess the advantage that they operate equally
well above or below the freezing point. Wet- and dry-bulb hygrometers
(or " psychrometers ") are ordinarily used for routine observations. They
depend on the accurate determination of the depression of the wet bulb
reading below the dry bulb and give sufficient precision for ordinary needs.
HYGROSCOPE
116
Their accuracy is considerably increased if arrangements are made, as in
the Assmann PSYCHROMETER, for " aspirating " the air at a known speed.
Prof. G. M. B. Dobson and Mr. A. W. Brewer have recently devised a
hygrometer for use in air at a temperature below 32° F. on aircraft making
meteorological ascents. It measures the frost point of the air by the deposition
of ice from a jet of air upon the cooled surface of an aluminium thimble.
The flow of cooling liquid inside the thimble is controlled by the observer so
that the ice deposit remains constant. The temperature of the thimble is
then measured by an electrical resistance thermometer. It was by means
of this instrument that the extreme dryness of the lower STRATOSPHERE was
discovered by A. W. Brewer. See STRATOSPHERE.
Hygroscope.—An instrument for showing whether the air is dry or damp,
usually by the change in appearance or dimensions of some substance.
Hygroscopes are frequently sold in the form of weather predictors, e.g.
" weather houses " in which the appearance of the " old man " or the " old
woman " is determined by the twisting and untwisting of a piece of catgut
in response to changes of humidity.
Hypsometer.—Literally, an instrument for measuring height (Gr. Hupsos).
The term is, however, applied exclusively to an instrument in which water is
boiled under conditions which make it permissible to assume that the
temperature within the vessel is, accurately, the boiling point of water. The
boiling point of water depends on the atmospheric pressure in accordance with
the following table :—
Boiling point,
Degrees absolute
374
373
372
371
370
369
368
367
366
365
364
Pressure
Millimetres of mercury at
0° C., sea level, latitude 45°
787-67
760-00
733-16
707-13
681-88
657-40
633-66
610-64
588-33
566-71
545-77
Millibars
1050-12
1013-23
977-45
942-74
909-08
876-44
844-79
814-10
784-36
755-54
727-62
Consequently, a measurement taken by means of a thermometer placed
in the hypsometer serves to determine the pressure. If the measurement is
made on a mountain and the pressure has been determined at some lower
level, the height of the station can be calculated. Everything depends upon
the accuracy of the thermometers. To obtain the height to within ten feet
it must be possible to measure temperature to within one-hundredth of a
degree. This requires very accurate thermometers combined with skilful
manipulation. Nevertheless, the compactness of a hypsometer gives the
instrument an advantage over the fragile and cumbersome mercurial
barometer in journeys of exploration.
If the atmospheric pressure is known from the reading of a good barometer,
a hypsometer may be used to determine the error at the boiling point of a
thermometer, provided of course, that the boiling point lies within the range
of the thermometer.
Ice.—Owing to the large amount of heat absorbed in melting (80 CALORIES
for one gramme melted) a mass of ice represents a powerful reservoir of cold.
Masses of ice or snow can attain to such dimensions in nature that the heat
absorbed during melting is of climatological importance. An excellent
example is furnished by the icebergs observed by Antarctic explorers. The
largest of these appear to be portions of the great Ice Barrier that have
117
ICE AGE
broken away during the summer months. They are generally several
hundred feet thick and may exceed 20 miles in length. The amount of heat
required to melt one 20 miles long, 5 miles broad and 600 feet thick would
be sufficient to raise the temperature of the air over the whole British Isles
from the ground up to a height of 1 kilometre (3,281 feet) by over 40° C.
When ice forms upon a pond during frosty weather the cooled water
at the surface is continually replaced by warmer water from below until
the whole mass has fallen to 277°A. (39° F.), which is the temperature at
which water has its greatest density. The surface can then cool undisturbed
until the freezing point is reached and ice begins to form. The formation of
ice in rivers is a more complicated question ; in cold countries three kinds
of ice are produced (1) sheet ice which forms on the surface of the water,
first of all near the banks extending gradually towards the centre (2) FRAZIL
ICE and (3) ground ice, both of which form in the rapidly-moving stream in
the centre of the river in very cold weather. Ground ice (or anchor ice)
forms at the bottom, adhering to rocks and other substances in the river
bed. It often rises to the surface dragging with it masses of rock ; it can be
very destructive to river structures.
When the sea freezes the crystals formed contain no salt but cannot
easily be separated from the brine which is mixed up with them, consequently
the water obtained by melting genuine sea ice is salt. When, however, this
ice forms hummocks under the action of pressure the brine drains out and
leaves pure ice. Newly-formed sea ice has a surprising degree of flexibility,
due to the fact that the crystals are separated by layers of brine or salt,
and even when it is several inches thick the surface can be moved up and
down unbroken by a swell. As the thickness increases this can no longer
happen, and the sheet is broken up into pieces, which grind together and
soon form the beautiful " pancake ice " familiar to polar explorers.
Some beautiful illustrations of ice forms have been given by Wright and
Priestley in their book on the British Antarctic Expedition.*
A. Maurstad in "Atlas of sea ice "f has suggested a terminology for use
in describing the various forms of sea ice. This atlas was adopted by the
International Meteorological Committee at Warsaw in 1935 for international
use.
The properties of ice depend very largely on its mode of formation.
H. T. BarnesJ gives the mean value of the DENSITY of ice formed from boiled
water as -91676, but the density of natural ice appears to be about -9170
decreasing slightly with age. ICEBERGS, owing to the amount of included
air, may have a much lower density. The coefficient of linear expansion
of ice at — 10° to 0° C. lies between -000050 and -000054 ; its specific heat
at 0° C. is about • 5.
The great ice masses of the Arctic Ocean and the Greenland Seas exercise
an appreciable influence on the weather of north-west Europe.§ The most
important effect for the British Isles is the tendency for much ice in spring
and early summer to be followed by stormy weather in late autumn and
winter.
Ice Age.—A geological period in which great glaciers and ice sheets
cover large parts of the continents, reaching the sea in places and lowering
the temperature of the oceans. The latest began early in the Quaternary
Period, when the ice covered much of Europe and North America ; the
present ice sheets of Greenland and the Antarctic are relics of this Ice Age.
Other great Ice Ages occurred in the Permo-Carboniferous, when the ice
sheets developed principally within the present tropics, in South America,
Africa, India and Australia ; and in the earliest known geological period,
the Archaean._______________________________________
* Wright, C. S. and Priestley, R. E.; " British (Terra Nova) Antarctic Expedition
1910-13. Glaciology." London, 1922.
t Oslo, Geophys. Publ., 10, No. 11, 1935
j Ottawa, Trans. roy. Soc. Can. Series 3, 3, 1909, pp. 4-6.
§ Brooks, C. E. P. and Quennell, W. A.; London, Geophys. Mem., No. 41 1928
ICEBERG
118
Iceberg.—A mass of land ice broken from a glacier or a barrier and
floating in the sea. The glacier bergs are greenish in colour, irregular in
shape and may be broken by crevasses. The bergs which are broken off
from an ice barrier are white, rectangular in shape and often very large ;
they are usually described as " tabular" and are characteristic of the
Antarctic. Another type of berg which is not a true iceberg is composed,
at least above sea level, of neve or compacted snow ; these bergs have much
the appearance of tabular bergs, but float with a greater proportion above
the sea surface than true icebergs.
With favourable winds and currents, icebergs drift into latitudes of
40-50°. Icebergs decrease in size by melting, erosion and calving. Calving
often disturbs the equilibrium and causes the iceberg to roll over. Melting
takes place most rapidly on the water line, and in warm water from below
when the iceberg calves frequently. Erosion is caused mostly by the waves
and the swell, but also by the rain. H. T. Barnes' experiments show that
iceberg ice is as pure as distilled water, containing only 4 parts of solid per
million. Icebergs also contain a considerable quantity of air, ranging in
various specimens examined from 7 to 20 or 30 per cent.
Iceblink.—The whitish glare in the sky over ice which is too far away to
be visible.
Ice Day.—An ice day is defined as a period of 24 hours beginning normally
at 9 h. G.M.T. on which the maximum temperature is 32° F. or less.
Ice, formation on aeroplanes.—In certain conditions ice may form on the
wings, propellers and other parts of an aircraft in flight and so affect its
aerodynamic characteristics that continued flight is either seriously impeded
or has to be abandoned. The four main types of ice formation may be
summarised in order of increasing danger (a) a white semicrystalline coating
of ice which has little effect on flying. It generally forms in clear air when a
cold aircraft enters warmer and damper air during a rapid descent and is due
to condensation of water vapour direct into ice crystals (see HOARFROST) ; (6)
a light white opaque deposit which forms when flying in filmy clouds
consisting of small supercooled water drops (see RIME); (c) a transparent or
translucent coating of ice having a glassy surface appearance which forms
within dense clouds consisting of large supercooled water drops ; and (d) a
heavy coating of clear ice which forms when rain falls on an aircraft whilst
flying in a layer of the atmosphere whose temperature is below freezing
point (see GLAZED FROST).
The physics of ice formation indicate that when a supercooled water drop
impinges on an aircraft, part of the drop freezes immediately at the leading
edge whilst the remaining part of the drop freezes subsequently. In the
case of a small drop the final freezing occurs quickly giving opaque ice on and
near the leading edge. For a large drop the final freezing is not quite so rapid
and the drop has opportunity to spread over the wing. This leads to the more
dangerous type of clear ice which not only builds up outwards from the
leading edge but also spreads back over the wings. For ice accumulation to
become dangerous it is necessary that there should be abundance of large
water drops at air temperatures below the freezing point. These conditions
are met with only in clouds, rain, sleet, or rain and hail.
Observations made in England show that ice formation may occur in any
type of cloud, except members of the cirrus group which are composed of very
small ice crystals, provided the temperature in the cloud lies within the
range 10° F. to 32° F. It is occasionally experienced at temperatures below
10° F. but is usually not then heavy. The cloud in which it is most frequently
encountered in winter is stratocumulus but the heaviest deposits occur in
cumulus and cumulonimbus. Stratocumulus is a " layer " type of cloud from
which rain does not normally fall and it is usually possible to fly in clear air
either above or beneath it. Cumulus and cumulonimbus are " heap " clouds
and as a rule there is no difficulty in avoiding them. Cumulonimbus is often
associated with heavy showers with thunder and lightning. Ice formation
is comparatively rare in altostratus clouds probably because they are
frequently composed of ice crystals.
119
INSOLATION
The worst type of ice formation, GLAZED FROST, is associated with the
passage of fronts, chiefly warm fronts, in winter. It is comparatively rare in
this country but more frequent on the continent and in North America. The
average height of the freezing point level in the British Isles varies from about
1,800 ft. in winter to 10,000 ft., in summer and represents the average lowest
levels where ice formation may be expected. On a few occasions during every
winter the ice zone extends down to ground level, and even in the warmest
part of the summer it has been known to extend below 5,000 ft.
Ice Saints.—St. Mamertius on May 11, St. Pancras on May 12 and St.
Gervais on May 13 are known on the continent as the " cold Saints' Days."
It is said in France that these three days do not pass without a frost. From
observations at selected stations in Scotland from 1857-66 Buchan found
a cold period from May 9-14. For further information see London,
Meteorological Magazine, 57, 1922, p. 177 and 63, 1928, p. 93.
Ice Sheet.—A large area of land ice, with a dome-shaped, almost level
surface, which almost completely covers the land on which it rests, so that
precipitation of snow takes place directly on the surface of the ice. The best
known and largest ice sheets at present existing are those of the Antarctic
and Greenland.
Index.—The pointer or other feature in an instrument whose position
with regard to the scale determines the reading. The term is also sometimes
applied to the fixed mark which constitutes the zero, e.g. in the term " indexerror." See ERROR.
Index Correction.—The quantity which (with proper sign) has to be added
to an instrumental reading to correct for index error (see ERROR). It is of the
same magnitude as the index error, but of the opposite sign.
Index Error.—See ERROR.
Indian Summer.—A warm, calm spell of weather occurring in autumn,
especially in October and November. The earliest record of the use of the
term is at the end of the eighteenth century, in America, and it was introduced
into this country at the beginning of the nineteenth century. There is no
statistical evidence to show that such a warm spell does tend to recur each
year.
Insolation.—A term applied to the solar radiation received by terrestrial
or planetary objects (Willis Moore).
The amount of solar radiation which reaches any particular part of the
earth's surface in any one day depends upon (1) the SOLAR CONSTANT, (2)
the area of the intercepting surface and its inclination to the sun's rays,
(3) the transparency of the atmosphere, and (4) the position of the earth in its
orbit. The following table compiled from Angot* assumes the atmosphere to
be perfectly transparent.
Calculated Insolation reaching the Earth.
|H
Latitude
3
0
<—i
2
fe
0)
*
§
=5
0,
N.
90°
80°
60°
40°
20°
Equator
0-0 0-0 1-9 17-5
0-0 0-1 5-0 17-5
3-0 7-4 14-8 23-2
12-5 17-0 23-1 28-6
22-0 25-1 28-6 30-9
29-4 30-4 30-6 29-6
S.
20°
40°
60°
80°
90°
33-8
34-8
33-0
34-2
34-7
ctf
s
S
i—§>
3
3
3
<
31-5
30-5
30-2
32-4
31-8
28-0
36-4
35-8
33-2
33-8
32-0
27-1
32-9
32-4
31-1
32-8
31-8
27-6
21-1
20-9
24-9
29-4
30-9
28-6
^
"a,
1
V
in
!
"o
0
|H
<u
,a
1o
Z
jz8
S
80)
Q
4-6 0-0 0-0 0-0
7-4 0-6 0-0 0-0
16-7 9-0 3-8 1-9
24-3 18-4 13-4 11-1
28-9 25-8 22-5 20-9
30-1 30-2 29-5 28-9
32-2 29-0 24-9 21-2 19-6 20-5 23-7 27-7 31-1 33-3
30-4 23-9 17-4 12-5 10-4 11-6 15-8 21-9 28-5 33-6
25-3 16-0 8-1 3-3 1-7 2-7 6-5 13-6 22-6 31-1
20-5 6-3 0-3 0-0 0-0 0-0 0-0 3-8 16-0 31-0
20-7 3-2 0-0 0-0 0-0 0-0 0-0 1-0 15-6 31-5
34-1
36-0
35-3
38-1
38-7
^
o
>
145-4
150-2
199-2
276-8
331-2
350-3
331-2
276-8
199-2
150-2
145-4
La distribution de la chaleur £ la surface du globe." Ann. Bur. mil., Paris, 1883.
INSTABILITY
120
The unit is the amount of energy that would be received on unit area
at the equator in one day, at the equinox, with the sun at mean distance, if
the atmosphere were completely transparent. The solar constant being taken
as 1 -94 cal./cm. 2/min., Angot's unit is found to be 889 cal./cm. (The numerical
factor which is introduced is 24 X 60 /n.)
Instability.—Any system can remain in equilibrium when the forces
acting upon it balance each other. If further, when the system is disturbed
from its equilibrium position and then left to itself, it returns to its original
state, it is said to be in stable equilibrium. If when disturbed and left to
itself it stays in the disturbed position it is said to be in indifferent
equilibrium. If when disturbed and left to itself, it goes still further from
its original state, it is said to be in unstable equilibrium. Stability and
instability thus have to be defined in terms of the effects of small disturbances.
A pencil standing on end is stable, since it is not upset by small disturbances,
though it falls down when disturbed through a not very large finite angle.
In meteorology we are mainly concerned with the vertical stability of air
masses, and the customary treatment of the question lays down the condition
of stability as the possession of a LAPSE rate not exceeding the ADIABATIC.
When the LAPSE rate grows to a value exceeding the adiabatic, convection
begins, the convection tending to reduce the lapse rate to the adiabatic value
once more. It is not unusual however to find in practice lapse rates exceeding
10 or even 20 times the adiabatic for a short distance from the ground upwards
in strong sunshine. It thus appears that the phenomena are likely to be more
complicated than the assumptions usually made in discussing this topic.
The discussion above refers to statical equilibrium. When a system in
motion is slightly disturbed, if the result is to produce small oscillations
superimposed on the original motion, the system is dynamically stable.
If the result is to cause an increasing departure from the original motion,
the system is said to be dynamically unstable.
Instability Shower.—See SHOWER.
Interpolation.—When a varying quantity has been observed at certain
intervals of time, or of some other independent variable, the evaluation
of values appropriate to intermediate stages of the independent variable is
called interpolation. When a curve can be drawn to represent the observations
with reasonable accuracy, the values for intermediate values of the independent
variable can be read off from the curve. Numerous formulae have also been
developed for the same purpose. These might be used for example in deducing
the intermediate values of a function from values tabulated for definite
intervals. On account of its utility in such cases, interpolation has been
described as " reading between the lines of a mathematical table."
Intertropical front.—The surface of separation between the wind circulations
proper to the northern and southern hemispheres. Over the Atlantic and
Pacific Oceans, where it is closely related to the DOLDRUMS, it is the boundary
between the north-east and south-east TRADE WINDS. Over the continents
it is replaced by the boundary between other wind systems with components
directed towards the equator, e.g. in Africa between the HARMATTAN and the
south-west MONSOON. The FRONT moves northward in the northern summer
and southward in the southern summer, but its average position is north of
the equator. Over the oceans the range of movement is small, but over the
continents it may be very large.
Inversion.—An abbreviation for " inversion of temperature-gradient "
(see GRADIENT). The temperature of the air generally gets lower with
increasing height but occasionally the reverse is the case, and when the
temperature increases with height there is said to be an ".inversion."
There is an inversion at the top of a fog layer, and generally at the top
of other clouds of the stratus type. Inversions are shown in the diagram
of variation of temperature with height in the upper air, Fig. 26, by the
slope of the lines upwards towards the right instead of towards the left,
121
IRIDESCENCE OR IRISATION
which is the usual slope. In the TROPOSPHERE inversions do not generally
extend over any great range of height; the fall of temperature recovers
its march until the lower boundary of the STRATOSPHERE is reached. At
that layer there is generally a slight inversion beyond which the region is
isothermal, so far as height is concerned. For that reason the lower boundary
of the stratosphere is often called the " upper inversion." In some soundings
with ballons-sondes from Batavia the inversion has been found to extend
upwards for several kilometres from the commencement of the stratosphere.
It is usual, at least in temperate latitudes, to find an inversion somewhere
between the ground and a height of about a hundred metres at night, when the
sky has been clear for several hours and the wind has not been very strong.
During anticyclonic weather in winter this surface inversion may grow until
it extends to a considerable height and it may then persist for several days.
The dense fogs which characterise some cities in winter usually originate from
such an inversion. Alternatively, the accumulation of smoke overhead may
produce almost complete darkness at midday in a city if the inversion is
sufficient to prevent the smoke from dispersing. Fogs on land nearly always
imply the existence of an inversion. An inversion invariably means thermal
stability of the atmosphere and the absence of turbulence.
The term " counter-lapse" has been suggested as an alternative to
inversion.
Ion.—The name selected by Faraday for the component parts into
which a chemical molecule is resolved in a solution. Of the two components
one carries a positive charge the other an equal negative charge. The laws
of electrolysis indicate that these charges are of the same magnitude for
ions of all substances. Electric force causes the free ions charged with
positive electricity to move in one direction whilst those charged with negative
electricity move in the opposite direction. These movements constitute the
electric current through the solution.
Similarly a gas may conduct electricity by virtue of the presence of free
ions. The ions may be produced by combustion, by the action of radio-active
substances (like radium), by ultra-violet light, or by cosmic radiation.
The conduction of electricity through the atmosphere is now attributed to
the free ions which exist in it. (See ATMOSPHERIC ELECTRICITY.)
It is supposed that all matter is built up of units which have positive
or negative charges. The positively charged units are called protons, the
negatively charged ones electrons. In neutral matter the positive and
negative charges just balance, according to this hypothesis. A negative ion
is matter to which an extra electron is attached, a positive ion is matter which
has lost an electron. The ionic charge is always a multiple of the charge of an
electron, the value of which as determined by Millikan is 15 • 9 x 10~20 coulomb.
Iridescence or Irisation.—Words formed from the name of Iris, the rainbow
goddess, to indicate rainbow-like colours. In its more technical usage
iridescence refers to tinted patches generally of a delicate red or green,
sometimes blue and yellow, occasionally seen on high clouds. A brilliant
iridescent cloud of considerable area is " one of the most beautiful of sky
phenomena." The boundaries of the tints are not circles with the sun as
centre but tend to follow the outlines of the cloud. The iridescence is
probably due to diffraction by small waterdrops and the colours seen are
determined partly by distance from the sun, partly by the size of the drops,
whereas in the production of the corona the variations between the drops are
of little importance and the same tint is to be seen at the same distance from
the sun in any direction. The drops responsible for iridescence are very small
and probably supercooled well below the freezing point. Just as a HALO is
evidence for the presence of ice crystals so iridescence is evidence for the
presence of supercooled waterdrops.
In Norway, Prof. St0rmer has had photographs of iridescent clouds taken
with cameras at a considerable distance apart and has found the height of the
clouds to be about 20 Km., more than twice the height of ordinary cirrus clouds.
ISALLOBAR
122
Isallobar.—Isallobars are lines drawn upon a chart through places at
which equal changes of pressure have occurred in some period of time. They
are formed by plotting the changes in barometric pressure which have taken
place between two sets of observations. Lines of equal change, or isallobars,
are drawn to enclose regions of rising and of falling pressure. At the
Meteorological Office, the isallobaric chart is drawn each day from the obser­
vations taken at 7h. G.M.T. and the isallobars represent the differences
in pressure in half millibars between the readings of the barometer at 4h.
and 7h. The barometer is not actually read at 4h. but the rise or fall of
pressure can be obtained with sufficient accuracy from a barograph trace.
Generally the areas of rise or fall in an isallobaric chart are regular in forms.
Dr. Nils Ekholm,* by whom this method of plotting the pressure differences
was introduced, claimed that when successive charts of isallobars were drawn
the movements of the various groups of isallobars were on the whole more
regular than those of the isobaric groups. E. G. Bilham.f however, points
out, when the pressure distribution is accurately known at the beginning
and end of a time interval, that
" in the case of small depressions uncomplicated by other systems the isallobars
are merely a sort of composite picture of the two sets of isobars and no
additional information is imparted by them. . . . When the distribution
is complex, however, the isallobaric chart is of value since it performs the
function of separating the active from the quiescent features of the pressure
distribution."
Isallobaric Wind.—A component wind due to changing pressure gradients.
It is only on occasions when the pressure distribution is not varying that
the use of the gradient wind equation is strictly justified. The mathematical
expression for the wind in a changing pressure system includes, in addition
to the " GRADIENT WIND," a component directed at right angles to the
ISALLOBARS and proportional to the gradient of the isallobars, this component
being known as the isallobaric wind, and a term representing a harmonic
oscillation of the wind. The last mentioned term would be difficult to detect
because it should be damped out rather rapidly by surface frictional effects,
but important discrepancies which have on occasions been found when
attempts have been made to relate upper wind measurements to the pressure
gradient and the isallobaric gradient are of an order quite compatible with
the temporary existence of the initial oscillation.
The isallobaric component is relatively small, rarely exceeding 5 m./sec.;
but is dynamically important in problems of convergence and rainfall.
Isanomaly.—This word is a combination of the prefix iso and the word
ANOMALY, and is used of lines joining all points on a map or chart having
equal anomalies, or differences from normal, of a particular meteorological
element.
Isentropic.—Without change of ENTROPY, generally equivalent in meaning
to ADIABATIC. Isentropic surfaces in the atmosphere are surfaces of constant
POTENTIAL TEMPERATURE.
Isentropic Analysis.—An analysis, used particularly in the United
States, of the physical and dynamical processes taking place in the free
atmosphere on the basis of the location and configuration of the various
isentropic surfaces and distribution of atmospheric properties (e.g. HUMIDITYMIXING-RATIO) and motion on them. ISENTROPIC CHARTS are the main tool
in this work.
Isentropic charts.—Synoptic charts of atmospheric properties on specified
isentropic surfaces.
Iso.—A prefix meaning equal, extensively used in meteorological work,
in conjunction with another word, to denote lines drawn on a map or chart
to display the geographical distribution of any element, each line being drawn
• " Wetterkarten det Luftdruckschwankungen." Met. Z., Braunschweig, 21, 1904,
p. 345.
t " Isallobars of moving circular depressions." Geogr. Ann., Stockholm, 1921, H.4, p. 356.
123
ISOMERIC VALUES
through the points at which the element has the same value. The words
having this prefix are generally self-evident in meaning and can be interpreted
without any difficulty.
In 1889 the tentative use of the word isogram was suggested by Sir Francis
Galton as a generic term for all lines of this type. The name isopleth is literally
applicable for this purpose and is so used by many writers. The use of isogram,
however, seems advisable, in order to avoid confusion, as isopleth is often
used with a more specific meaning, viz., a line showing the variation of an
element with regard to two co-ordinates, such as the time of year (month) and
the time of day (hour).
Some of the more important examples of the use of iso are set out below.
Isobars :—Lines on a chart joining places of equal barometric pressure.
Isotherms :—Lines showing equal temperatures.
Isohyets.—Lines showing equal amounts of rainfall.
Isohels :—Lines showing equal duration of sunshine.
Isonephs :—Lines showing equal amounts of cloudiness.
Isobar.—The distribution of atmospheric pressure is indicated on a
weather map by means of isobars—these being lines of equal pressure or
lines along which the pressure is constant.
The atmospheric pressure at any given place is readily obtained by means
of a mercury barometer, but in order that the barometric readings from
different places may be comparable one with another it is necessary that
they be reduced to a common standard. Corrections are, therefore, applied
to reduce the readings to mean sea level (see REDUCTION TO SEA LEVEL).
On the charts used at the Meteorological Office in connexion with weather
forecasting, it is customary to plot the barometric readings to the nearest
tenth of a millibar and to draw the isobars for every two millibars, the isobars
being those of even number, i.e. 1000 mb., 1002 mb., 1004 mb., etc. It
seldom happens that an exact even millibar reading occurs at any station at
one of the fixed observation times, and in consequence it is necessary to find
the position of the even millibar readings by means of interpolation. When
there is a close network of stations and the pressure variations from place
to place are not too great, this is quite a simple matter, if it be assumed
that the barometer falls uniformly between two stations. By repeating this
process a series of positions for any given value is obtained and by drawing
a curve connecting these points the isobar for this value is formed. Isobars
even in complicated pressure systems can be drawn readily after some practice.
It will be seen that when travelling along an isobar pressure remains
continually higher on one side of the isobar than on the other, and that an
isobar must come back again to the place from whence it starts if a large
enough area be considered. This may be simply stated by saying that an
isobar must form a closed curve.
After the pressure readings have been plotted on a map and the isobars
drawn, it will be seen that the isobars are analogous to contour lines, the
areas of high pressure corresponding with the hills and those of low pressure
with the valleys.
Examples of different systems of isobars will be found in the illustrations
which accompany the articles on ANTICYCLONE, COL, DEPRESSION, SECONDARY,
TROUGH and WEDGE.
Isomeric Values.—The values obtained when the average monthly rainfall
amounts for any station are expressed as percentages of the annual average.
When the percentage values for a number of stations for any month are plotted
on an outline map an isomeric chart is obtained for the month, and the lines
drawn on such a chart to represent the distribution of the values are termed
" isomers " or lines of equal proportion. Isomeric charts of the British Isles
showing the proportion of the annual average rainfall falling in each month
of the year were published in London, Quart. J. R. met. Soc., 41, 1915, pp. 1-44
and British Rainfall, 1914, pp. 25-44.
ISOPYCNIC
124
Isopycnic.—Relating to field of mass. Term for surface of equal density.
See BAROCLINIC. Isopycnic charts of the globe for different heights in the
atmosphere are given in London, Quart. J. R. met. Soc., 50, 1924, pp. 37-41.
Isosteric.—Relating to field of mass. Term for surface of equal specific
volume. See BAROCLINIC.
Isotherm.—A line joining places along which the temperature of the air
or of the sea is the same. When the places are at different altitudes a
correction may be necessary on account of the general upward decrease of
temperature (see REDUCTION TO SEA LEVEL). The first isotherms were
constructed by A. von Humboldt in 1817. Isotherms of mean daily maximum
and mean daily minimum temperature are given in " The Book of Normals
of Meteorological Elements for the British Isles," Section III. Isotherms
for the globe for each month and the year are contained in the " Manual
of Meteorology," Vol. 2, by Sir Napier Shaw (Cambridge University Press,
2nd Edition, 1926).
Isothermal—of equal TEMPERATURE. An isothermal line is a line of
equal temperature, and, therefore, is the same as isotherm.
Isothermal is frequently used in meteorological writings on the upper
air for the so-called " isothermal layer " by which is meant the layer indicated
in the records of all sounding balloons, of sufficient altitude, by the sudden
cessation of fall of temperature with height and generally by a slight INVERSION
(see also GRADIENT) followed by practical uniformity of temperature.
The layer is not really isothermal, since it has a horizontal temperature
gradient. The word STRATOSPHERE was therefore coined by M. Teisserenc
de Bort, who was instrumental in discovering this region, while he gave the
name of TROPOSPHERE to the region below. These names have now been
generally adopted.
Jet Stream.—Intense winds blowing along a relatively narrow " tube " for
a great distance have been encountered in the upper TROPOSPHERE of temperate
regions. These are called jet streams. The wind velocity in the " stream "
may be 150 kt. The wind in a jet stream always blows from a westerly
point. The wind velocity falls off rapidly in all directions from the axis of
the jet.
Joule.—The unit of energy associated with the practical system of
ELECTRICAL UNITS.
1 joule = 1 watt-second = the energy converted to heat when a current
of one ampere flows through a resistance of one ohm for one second.
The joule is approximately the energy required to lift one kilogramme
through 10 centimetres ; it is equal to 107 ergs.
The 20° calorie, the heat required to raise the temperature of one gram
of water by 1° C. from 19£° C. to 20J° C. is equivalent to 4-18 joules.
Katabatic.—An adjective applied to winds that flow down slopes that
are cooled by radiation, the direction of flow being controlled almost entirely
by orographical features. Such winds are an expression of the downward
convexion of cooled air. Local cold winds are sometimes cited as examples
of katabatic winds when the katabatic process intensifies a wind that would
otherwise be the cold current of the rear of a depression, as in the BORA.
Kew Pattern Barometer.—A portable marine BAROMETER designed by
P.. Adie in 1854 for the Kew Committee of the British Association. The
scale is graduated to take account of changes in the level in the steel cistern
so that it is only necessary to read the top of the mercury column. The tube
is constricted to minimise PUMPING when the barometer is used at sea.
Similar barometers, known as " station " barometers, but without the
constriction, were subsequently adopted for use on land, and are regularly
employed at most British and Colonial meteorological stations.
125
LAG
Khamsin.—A southerly wind blowing over Egypt in front of depressions
passing eastward along the Mediterranean or north Africa, while the
pressure is high to the east of the Nile. As this wind blows from the interior
of the continent it is hot and dry, and it often carries a considerable quantity
of dust. It is supposed to be most prevalent for about 50 days from April to June.
Gales from S. or SW. in the Red Sea are also known as khamsin.
Kilowatt-hour.—The unit by which electrical energy is sold. This unit
has sometimes been referred to as a Board of Trade Unit but that name
is not to be recommended as confusion may arise with the " British Thermal
Unit " the heat required to raise the temperature of a pound of water by
1° F.
1 kilowatt-hour = 3-6 X 1013 ergs.
= 8-6 X 106 calories.
Kite.—An appliance which ascends into the air in virtue of the pressure
of a relative wind upon an inclined surface. The kite is held captive by means
of a cord or wire. It consists of a framework of wood or light metal, covered
with fabric, or other suitable material. The general form used for scientific
work is the box kite, invented by Hargrave; this form consists of a light
wooden framework, usually rectangular in cross section, covered with fabric
at either end for about one quarter of the length, the middle portion being
left free, thus forming rectangular cells. Kites are frequently flown in tandem,
and heights of over 20,000 ft. have been reached. A box kite of lozenge shape,
was used by W. H. Dines, a special form of meteorograph being designed for
use with it.
There are many forms of kites, from the Malay kite, with the attached
tail, to the large man-lifting kites used by Cody and others. Kites were
known to the Chinese at a very early date, and were flown upon festive occasions
High-altitude kite-flying necessitates the use of a thin steel wire.
Kite Balloon.—A captive balloon, elongated in form, and fitted with
fins or air ballonets at the rear end. The kite balloon is so rigged, that when
moored, it flies with the forward end tilted to the wind, thus, the underside
of the gas bag acts as a plane surface, tending to lift the balloon in the manner
of a kite.
The fins or ballonets are connected with an opening facing the wind, by
which they are kept inflated.
The observer's basket is suspended from the gas bag by means of cords.
Small kite balloons have been designed for carrying meteorographs. Unless
special precautions are taken, the steel cable constitutes a danger to aircraft,
as in the case of the kite. See BALLOON, KITE.
Kuro-Shiwo.—Variously translated " blue salt " stream or " black "
stream, a warm-water current of a characteristic dark blue colour the main
branch of which flows north-eastward along the south coast of Japan, and
continuing on this course gradually merges in the general eastward drift
of the North Pacific. This current is analogous to the Gulf Stream of the
Atlantic, carrying warmth to higher latitudes ; between it and the east coast
of Japan flows a cold current from the Bering Sea, analogous to the Labrador
current. The strength of the Kuro-Shiwo varies with the wind, increasing in
the (summer) season of the south-west monsoon and decreasing in the (winter)
season of the north-east monsoon. A branch of the Kuro-Shiwo sets into the
Japan Sea.
Labile.—A term sometimes used to denote unstable or neutral equi­
librium in the atmosphere, i.e. LAPSE RATE equal to or exceeding the
ADIABATIC. In German the word is used to denote unstable equilibrium only.
Lag.—The delay between a change in the conditions and its indication
by an instrument. It is the aim of instrument makers to reduce this interval
as much as possible, but a limit is often set by practical considerations such
as the need for robust construction. In marine barometers lag is introduced
LAND AND SEA BREEZES
126
deliberately to minimise " pumping." In Symons's earth thermometers lag
is introduced by immersing the bulb in paraffin wax, a poor conductor of
heat, so that the reading does not change perceptibly when the thermometer
is drawn up for reading into surroundings at a different temperature.
Land and Sea Breezes.—These are caused by the unequal heating and
cooling of land and water under the influence of solar radiation by day and
radiation to the sky at night, which produce a gradient of pressure near the
coast. During the day time the land is warmer than the sea and a breeze,
the sea breeze, blows onshore ; at night and in the early morning the land
is cooler than the sea and the land breeze blows offshore. The breezes are
most developed when the general pressure gradient is slight and the skies
are clear. In such circumstances the sea breeze usually sets in during the
forenoon and continues until early evening, reaching its maximum strength
during the afternoon ; the land breeze may set in about midnight or not until
the early morning. The land and sea breezes are much influenced by topo­
graphy and vary considerably from one part of the coast to another. In
the British Isles the pure sea breeze rarely exceeds force 3, but in lower
latitudes it may reach force 4 or 5. The land breeze is usually less developed
than the sea breeze. The sea breeze does not usually extend more than 15-20
miles on either side of the coast line, and often its extent is considerably
less. The depth of the current is shallow at its commencement, but in favour­
able circumstances it may exceed 2,000 feet, at the time of its maximum
development.
Lapse.—The term lapse was proposed by Sir Napier Shaw to denote the
decrease of temperature of the atmosphere with height. The lapse rate is
the fall of temperature in unit height, and is taken positive when the tem­
perature decreases with height.
When the temperature at different heights is represented diagrammatically,
height being conveniently measured along the vertical co-ordinate and tem­
perature along the horizontal axis, the condition of the atmosphere is repre­
sented by a curve which is sometimes called the " lapse line." Average
conditions in the atmosphere are represented by a straight line whose slope
corresponds to a lapse rate of about 0-6° C. per 100 metres.
The lapse rate of temperature is of fundamental importance in deter­
mining the vertical stability of the atmosphere (see ADIABATIC, INSTABILITY
and ENTROPY). When the lapse rate is negative the temperature increases
with height (see INVERSION). In the TROPOSPHERE this only occurs through
limited intervals of height.
Latent Heat.—The quantity of heat absorbed or emitted without change
of temperature during a change of state of unit mass of a material. The
latent heat of fusion of one gram of ice is 79-77 gram calories. The latent
heat of condensation of 1 gram of water vapour varies from 597 gram calories
at 0° C. to 539 gram calories at 100° C.
Latent Instability.—See INSTABILITY.
Latitude.—The geographical latitude is defined as the angular elevation
of the celestial pole above the surface tangential to the spheroid which
represents the earth. It is also equal to the angle between the normal to
this surface and the plane of the equator. The geographical latitude <j> at
any point differs only slightly from the geocentric latitude <}>', the latter
being the angle between the radius of the earth passing through the point
and the plane of the equator. The relation of (p and (j)' is approximately
represented by the equation (J>—$' = 68-8* sin 2<j>. Geocentric latitude is
found to be more useful than geographical latitude in seismology as the
calculation of distance from place to place is facilitated.
Astronomical latitude is defined as the elevation of the celestial pole
above the level of a mercury surface, or in other words as the angle between
the plumb line and the plane of the equator.
127
LIGHTNING
Law of Storms.—A nautical expression denoting the mariners' rules for
minimising the dangers of tropical cyclones, and especially for avoiding the
so-called " dangerous half " of the cyclone. These rules as used at the present
time were drawn up by Dr. C. Meldrum and Captain Wales.
Least Squares, Law of.—A law, based on the theory of errors, which states
that the best estimate of any quantity, observations of which are distributed
according to the normal law of errors, is that for which the sum of the squares
of the residuals of these observations is least. It may be applied to pairs of
related values xt ..... #n and y1 ..... y0 to obtain the constants of the
function y = f (x) which best expresses the relation between x and y, i.e.
that for which the sum of the squares of the differences between the observed
and computed values of y is a minimum.
Lenticular.—In shape like a lens or lentil. The shape1 is characteristic
of a type of cloud most often seen over hills or mountains, formed in a damp
layer at the crest of a stationary or slow-moving wave. The cloud is formed
by a large mass of clustered cloudlets and is apparently disposed horizontally.
It has well denned edges, a pointed end and a broad middle or base. The
cloud is usually of smooth appearance, but the cloudlets are often visible,
streaming through the cloud. Sometimes the side of the cloud has an appear­
ance not unlike that of stratified rocks. The lenticular clouds are frequently
associated with FOHN. Clouds of rather similar structure are the CAPS on
cumulus clouds, and the flat patches sometimes visible near cumulonimbus,
often appearing dark against a white background.
Leste.—A hot, dry, S. wind occurring in Madeira and northern Africa
in front of an advancing depression.
Levanter.—An easterly wind in the Straits of Gibraltar. It is most frequent
from July to October and in March. At Gibraltar it causes complex and
dangerous eddies in the lee of the Rock. With moderate winds a banner
cloud stretches out from the summit of the Rock a mile or more to leeward ;
when the wind reaches force 7 or over the cloud lifts and disappears.
Levanter is usually associated with high pressure over western Europe and
low pressure to the south-west of Gibraltar over the Atlantic or to the south
over Morocco.
Leveche.—A hot, dry, southerly wind which blows on the south-east coast
of Spain in front of an advancing depression. It frequently carries much dust
and sand, and its approach is indicated by a strip of brownish cloud on the
southern horizon.
Level.—A surface is level, if it is everywhere at right angles to the direction
of the force of gravity, which is indicated by the plumb-line. The word is
also used to indicate the height of a level surface above a standard such as
MEAN SEA LEVEL.
Lightning.—The flash of a discharge of electricity between two clouds
or between a cloud and the earth. A distinction is drawn between " forked "
lightning, in which the path of the actual discharge is visible, and " sheet "
lightning, in which all that is seen is the flash of illuminated clouds and which
is attributed to the light of a discharge of which the actual path is not visible.
Since the introduction of photography many photographs of lightning
have been obtained, and in general character they cannot be distinguished
from photographs of electric discharges of six inches or more in length which
are obtained in a laboratory, but the varieties of form of lightning discharges
are very numerous. Frequently a flash shows many branches, especially the
upper part of a flash between the clouds and the earth. Among a collection
of photographs thrown upon a lantern screen, W. J. S. Lockyer once interpolated
a photograph of the River Amazon and its tributaries, taken from a map,
and the photograph was accepted without comment as a picture of lightning.
LIGHTNING
128
The quantity of electricity which is discharged by a lightning flash has
been shown by C. T. R. Wilson* to be of the order 20 coulombs. A. Matthias,
who utilised an oscillograph with a very open time scale, 10 cm. to a second,
found at Wannsdorf near Berlin that a single lightning flash usually involved
about five partial discharges. The duration of a partial discharge was between
•0005 second and -01 second. The maximum interval between consecutive
partial discharges was 0 • 37 second. The same observations indicate that the
theory that the lightning discharge is oscillatory cannot be upheld.
The development of a lightning flash has been studied in South Africa
by Prof. B. F. J. Schonland and his colleagues with the aid of the camera
invented by Sir Charles Vernon Boys. This camera had two lenses which
were turned rapidly about a common axis. (See BOYS CAMERA.) Two photo­
graphs are taken simultaneously and, by comparison of the photographs, it is
possible to determine the sequence of events. It is found that a flash
begins with a faint light travelling down from the cloud in a jerky way
and leaving a fainter trail. At intervals the trail branches, the light
travelling simultaneously along different tracks. Eventually one of the
branches approaches the earth. Then a vigorous, much brighter, luminosity
travels rapidly up this branch, and the other branches light up in
succession. Subsequent strokes are not branched, but they have the same
double character, leader and return stroke. The time taken by the leader to
reach the ground is comparable with -01 sec. ; the return stroke covers the
same distance in about -0001 sec. This interpretation of the Boys photo­
graphs is in agreement with results obtained in the study of the "atmospherics "
which are heard on the wireless and which are evidence that electric waves
produced by lightning can travel right round the globe.
The sign of the electricity brought down by a lightning stroke which hits
an electric power line has been determined occasionally. There is a great
preponderance of negative strokes. Laboratory experiments by Alibone
+ +.+ + 4- -4-
t»\t
Reproduced by permission from London, Proc. Toy. Soc., A, 164,1938, p. 134.
FIG. 23.—The development of a flash of lightning.
* London, PMlos. Trans., A., 221, 1920, p. 73.
129
LONG-RANGE FORECAST
indicate that this is probably because negative lightning is strongly attracted
by elevated structures which are apt to carry streamers of St. Elmo's fire.
There is no such attraction for positive lightning which is more likely to go
straight to earth.
Line-Squall.—Squalls may occur simultaneously along a line, sometimes
300 or 400 miles in length, advancing across the country, and to such
phenomena the name " line-squall " is applied.
The passage of a line-squall at a ground station in a country like the
British Isles is marked by characteristic happenings which include all or some
of the following :—
An arch or line of low black cloud of definite straightness.
A sudden or rapid rise of wind velocity.
A sudden or rapid change of wind direction.
A sudden or rapid drop in temperature.
A rapid rise in barometric pressure.
Heavy rain or hail (occasionally snow).
Thunder and lightning.
It is the first and second of these characteristic happenings that, combined,
have given the special name. In the upper air the passage of the squall is
marked by dense cloud and very violent vertical disturbances.
The fundamental physical fact underlying the phenomena is that suddenly
a distinctly colder body of air replaces a warmer current, and this sudden
replacement is brought about when the COLD FRONT of a depression passes.
The advancing cold current pushes itself wedge-wise under the warmer air
and causes that warmer air to rise up an oblique plane ; this ascent is sufficient
to produce the cloud and precipitation mentioned, whilst the rise in pressure
and the drop in temperature are the results of the following cold air.
In the British Isles the warm current in front of the squall is usually
from a southerly or south-westerly point and moist. In some parts of the
world it is dry and in these latter cases the squall may pass without rain
or even without cloud ; the dryness means that a great ascent has to be
carried out before saturation, and hence cloud formation, is attained and
sufficient ascent may not occur.
Line-squalls usually cross the British Isles in an easterly direction. Their
speed of advance can be forecast with considerable certainty for in active
systems any given part of a cold front advances with a velocity which is
approximately equal to the component, perpendicular to the front, of the
gradient wind in the cold (polar) air in its rear.
The life of a line-squall is often 24 hours or more.
Local Time.—Time reckoned from the epoch noon which at any place is
the time of transit of either the true sun or mean sun (according as local
apparent or local mean time is being used) across the meridian. A sundial
and a sunshine recorder both indicate local apparent time (see TIME).
Longitude.—The intersection of a plane through the axis of rotation of
the earth with the earth's surface is called a meridian. The longitude of
any place is the angle between the meridian through that place, and a standard
meridian. The standard meridian is usually taken to be the meridian of
Greenwich.
Long-range Forecast.—A forecast for a period of a week or more; a
forecast for three or more months is, however, generally called a " seasonal
forecast." A wide variety of methods for long-range and seasonal forecasting
have been tried in different parts of the world, including :—
(1) Influence of variations of solar radiation on subsequent weather.
(2) Relations between weather in successive seasons in the same or
different parts of the world.
(3) Periodicities.
(4) Methods based on the generalised weather situation over a period
of time.
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LOOMING
130
Variations of solar radiation have been used by C. G. Abbot and
H. H. Clayton as the basis of forecasts for a week or more in North and
South America. The data are the daily measurements of the SOLAR
CONSTANT by the Smithsonian Institution, but there is some doubt as to the
significance of these values.
Relations between weather in different seasons.—This method was first
adopted in India for forecasting the MONSOON rainfall and has been developed
by Sir Gilbert Walker as a basis for seasonal forecasting (or " foreshadowing ")
in other parts of the tropics. He formed series of data for places in all parts
of the world, mostly means for three-monthly seasons, and correlated each
of these with all the others for the same season and for three and six months
before and after. From these CORRELATION coefficients he is able to derive
regression equations, connecting the element to be forecast with preceding
conditions in the same or other parts of the world.
In the United States sea temperatures in the North Pacific are used as
the basis for seasonal forecasts of rainfall in California, and the effect of the
temperature of the Gulf Stream on the weather of the eastern coasts is being
studied.
Periodicities in weather have frequently been used as the basis for
forecasts over extended periods, but have generally proved untrustworthy.
Weather cycles are not true periodicities in the mathematical sense, but are
subject to variations of phase, amplitude and length, or may appear and
disappear suddenly. Cycles appear to be more regular in tropical than in
temperate regions and have been used to forecast the seasonal rainfall of
Java with moderate success.
The generalised weather situation is that given by weather maps repre­
senting average conditions over a period of time, from 10 days to a month.
This smoothes out temporary features such as rapidly moving secondary
depressions and emphasises persistent or structural features such as slowmoving anticyclones and centres of cyclonic activity. In Germany, F. Baur
bases 10-day forecasts on the analysis of a succession of charts for overlapping
10-day periods by a combination of synoptic and statistical methods. In
Great Britain monthly charts of deviations of pressure from normal over the
northern hemisphere are being studied by synoptic methods.
Looming.—A nautical expression for an indistinct enlarged appearance
of any object, particularly during the prevalence of slight fog. The expression
is not used for unusual visibility by reason of which distinct objects appear
nearer. The term is also used for the optical phenomenon in which objects
normally below and beyond the horizon become visible.
Low.—Used to denote a region of low pressure, or a DEPRESSION, in the
same way as " high " is used for a region of high pressure or anticyclone.
On weather charts the central part of a depression is usually indicated by
the word " low."
Lowitz, Arcs of.—On rare occasions arcs slightly concave towards the sun
extend obliquely downwards and inwards from the parhelia of the 22° halo.
They are formed by refraction through ice crystals oscillating about the
vertical.
Luminous Night Clouds.—See NOCTILUCENT CLOUDS.
Lunar.—Dependent upon luna, the moon ; thus a lunar rainbow is a
rainbow formed by the rays of the moon, a lunar cycle a cycle dependent
upon the moon's motion. A month is really, from its name, a lunar cycle,
but the introduction of a calendar month makes it necessary to draw a
distinction between it and the lunar month, which is the period from new
moon to new moon. In astronomy it is called the synodic month, and is
equal to 29-5306 days on the average. See CALENDAR.
131
MAGNETOGRAPH
Lustrum.—A period of five years, which is commonly used for grouping
meteorological statistics which extend over a long period of years. In the
Latin original the word meant a purifactory ceremony performed after the
quinquennial census (Oxford Dictionary).
Mackerel Sky.—A sky covered with CIRROCUMULUS or ALTOCUMULUS
clouds arranged in somewhat regular waves and showing blue sky in the gaps.
Maestro.—A NW. wind in the Adriatic. It is most frequent on the western
shore and in summer ; it is strongest on the west sides of depressions. NW.
winds in other parts of the Mediterranean, notably in the Ionian Sea, and
on the coasts of Sardinia and Corsica are also known as Maestro.
Magnetic Character.—It has been the practice for many years at magnetic
observatories to divide days into three classes, "0," " 1 " and " 2," according
to the degree of disturbance ; " 0 " representing the days with a fairly normal
diurnal variation of magnetic force (see TERRESTRIAL MAGNETISM) and
most free from irregular movements, and " 2 " representing the days most
disturbed. The classifications so made are communicated quarterly to the
Royal Netherlands Meteorological Institute at De Bilt, where the data are
employed in accordance with an international agreement, in making an
official selection from each month of five of the quietest and five of the most
disturbed days.
Magnetic Storm.—A state of large irregular disturbance or fluctuation,
world-wide in extent, of the earth's magnetic field, the normal quiet-day
diurnal changes being largely or entirely masked and replaced by others
with special characteristics. The duration may vary from a few hours to a
few days. Some storms begin abruptly (a " sudden commencement ") and
apparently simultaneously, at least to within the limits of accuracy of measure­
ment of the autographic records, at all places. The intensity of disturbance
is greater in high than in low latitudes ; in middle latitudes the absolute
range of variation in a force component of the earth's field amounts to a
very few per cent, of the normal value of the component. A conspicuous
feature of most storms is the depression in the value of horizontal magnetic
force after the early stages of the storm. Storms may occur at any time of
the year and at any phase of the solar cycle ; but the frequency of occurrence
of highly disturbed conditions is greater near the equinoxes than near the
solstices, and greater in years of sunspot maximum than in years of suns pot
minimum. There is a marked tendency for magnetically disturbed conditions
to recur at intervals of about 27 days (the solar rotation period), although a
large disturbance may not be followed by another, or even by any noteworthy
disturbance, 27 days later. Other conditions being favourable, AURORA is
usually observed from places in comparatively low latitudes on occasions of
magnetic storms. On such occasions large and irregularly varying earth
currents cause interference with telegraphic communication. Definite effects
in radio-wave propagation have been observed during magnetic disturbances,
and it has recently been established that bright eruptions in the solar chromo­
sphere cause simultaneous radio fade-outs and distinct terrestrial magnetic
effects. See also TERRESTRIAL MAGNETISM.
Magnetogram.—The record from a MAGNETOGRAPH.
Magnetograph.—An instrument for obtaining continuous records of the
variations of the earth's magnetic field. The usual type includes three magnets,
delicately suspended with their axes horizontal. One, with its axis in the
magnetic meridian, records the changes of DECLINATION (D) ; a second is
perpendicular to the meridian and shows the fluctuations of horizontal
force (H) ; the third can move in the vertical plane and records the variations
of vertical force (V). Some forms are modified to give the geographical
horizontal components (N. and W.).
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E 2.
MAGNETOMETER
132
To each magnet a mirror is attached which reflects light from a fixed
lamp on to a rotating cylinder covered with photographic sensitised paper,
the cylinder normally rotating once in 24 hours, although in the " Quick
Run " type the period is much less.
In the " la Cour " instrument a system of prisms is incorporated in the
optical system in such a way that if the primary light spot moves off the
paper during a severe magnetic storm another spot comes on about 10 cm.
from the original. Very large variations can be recorded with this form of
instrument.
A fixed mirror is added below each magnet and light reflected from this
provides a straight line on the MAGNETOGRAM from which the curves can be
measured to obtain absolute values of the magnetic elements. The value
equivalent to the position of the base line is obtained by comparison of the
magnetogram with determinations from an absolute MAGNETOMETER.
Magnetometer.—An instrument for obtaining absolute measurements of
the earth's magnetic field. The elements usually measured are D, H and I
(the dip of a freely suspended magnet) ; other components are calculated
from these. For details of construction and principles involved in the use
of magnetometers, see " Dictionary of Applied Physics ", Vol. II, pp. 528-^40.
Mamtnatus.—The lower surface of certain cloud sheets sometimes has an
udder-like or mammillated appearance. This structure is distinguished by
the adjective " mammatus " or the prefix mammato, e.g. mammatocumulus.
Manometer.—An instrument for measuring differences of pressure.
Ordinarily the weight of a column of liquid is balanced against the pressure
to be measured. The mercurial BAROMETER is, therefore, a form of
manometer.
Mares' Tails.—The popular name for tufted CIRRUS clouds.
Maritime Polar Air.—See POLAR AIR.
Marsden Square.—Mainly used for identifying the geographical position of
meteorological data over the oceans. System devised by Marsden in 1831
who divided a Mercator chart of the world into squares of 10° LATITUDE by
10° of LONGITUDE. Each square is arbitrarily numbered, and is again sub­
divided into 100 one-degree squares which are numbered from 00 to 99 so
that given the position of the square the first figure of the sub-square denotes
the latitude and the second figure the longitude.
Maximum.—The highest value reached by any element in a given period.
For example, the highest temperature recorded during the 24 hours is the
maximum temperature for that period. See EXTREMES.
Mean.—The mean value of a set of numerical quantities is the average
value determined by adding them together and dividing the result by the
number of quantities. In some cases there may be an ambiguity unless the
context makes it clear as to how the numerical quantities are classified.
For example, the mean temperature of the atmosphere extending above a
certain place might indicate the arithmetical mean of the temperatures
taken at equal intervals of height or at equal intervals of pressure going
upwards. See also NORMAL, AVERAGE.
Mean Sea Level (M.S.L.).—Is defined as the average level of the sea as
calculated from a long series of observations obtained at equal intervals of
time. On Ordnance Survey maps of the British Isles the heights of places
are given above Ordnance Survey Datum, which was formerly taken as mean
sea level at Liverpool; of recent years mean sea level at Newlyn, Cornwall,
has been taken as this standard.
Median.—When a series of observations is arranged in order of magnitude,
the central value (if the number of observations is odd) or the mean of the
two central values (if the number is even) is termed the " median ". If any
observation is taken at random, there is an even chance that it will be above
or below the median. It may differ appreciably from the MEAN, in which
case the FREQUENCY distribution is said to be skew.
133
METEOR
Megadyne.—One million dynes. A pressure of a megadyne per square
centimetre is equal to one bar or 1,000 millibars. See DYNE.
Meniscus.—The curved upper surface of liquid in a tube. In the case of
water, the meniscus, when viewed horizontally against the light, appears as a
dark belt. The upper edge represents the highest point to which the water
is drawn up against the glass, and the lower edge the lower part of the surface
out in the middle of the tube. When the tube is broad like the measuring
glass of a rain-gauge, the bottom edge should be used in reading. Mercury
has a convex upper surface and in the case of the barometer the index is
adjusted to the top of the meniscus.
Mercator's Projection.—See PROJECTION.
Mercury.—Mercury is a metallic element of great value in the construction
of meteorological instruments. In the mercurial barometer its great density
enables the length of the instrument to be made moderate, while the low
pressure of its vapour at ordinary temperatures makes possible a nearly
perfect vacuum in the space above the top of the barometric column. In
the mercury thermometer there is no risk of condensation in the upper end
of the ntem, as in the case of the spirit thermometer.
Specific gravity = 13-5951 at 0° C.
Specific heat
= 0 -0335 at 273° A.
Vapour pressure = 0-00021 mb. at 0° C.
= 0-00343mb. at 30° C.
Freezing point = 234-2° A.
Coefficient of expansion = -0001818 per ° C.
Meteor.—A meteor or shooting star is a fragment of solid material entering
the upper regions of the atmosphere from outer space and visible by its
own luminosity. The luminosity is attributed to incandescence due to the
compression of the air in front of the meteor. A large meteor may have a
luminous trail that persists for half an hour or longer. The majority of meteors
are exceedingly small, comparable with grains of sand, but large bodies—
meteorites, aerolites or siderites—fall to the ground occasionally. There is
no generally accepted theory of the origin of meteors.
Accurate determinations of the track of the meteor by reference to the
constellations by observers in different parts of the country may enable
the height of the track to be determined. Usually a meteor becomes luminous
between 150 and 100 Km. above ground and disappears at about 80 Km.
It is remarkable that very few disappear at heights between 70 and 50 Km.
Most of those which get below 70 Km. get below 50 Km.
According to the theory of meteors worked out by Lindemann and
Dobson* the density of the air and therefore its temperature can be estimated
from the records of the velocity of meteors, their brilliance and their points
of appearance and disappearance. The theory indicates that at 60 Km.
and perhaps lower the atmosphere is at a high temperature, at least as high
as that near the ground. See AUDIBILITY ; OZONE.
Lindemann and Dobson* accept the explanation that " the meteor
appears when a cosmic particle of matter, moving at a sufficiently high speed
relative to the earth, becomes heated by atmospheric friction to such a
temperature that it evaporates ". The friction acts, however, not so much
by the rubbing of the air streaming past the meteorite as by building up of
a cap of air heated by compression. The temperature of the particle cannot
exceed, but must be somewhat lower than the temperature of this heated
cap but the cap itself does not seem to be incandescent.
* LINDEMANN, F.A. AND DOBSON, G.M.B. ; A theory of meteors, and the density and
temperature of the outer atmosphere to which it leads. Proc. roy. Soc., London, A, 102
1923, p.411.
'
METEOROGRAPH
134
The appearance of a large meteor is followed occasionally by sounds
usually known as " detonations." The sounds are probably due to the
waves created by the meteor making its way through the air, not to the
explosive breaking up of the meteor. The sounds can be heard occasionally
at a distance of 50 miles from any part of the visible path of the meteor. In
some cases a zone of silence and an outer zone of audibility have been found.
To explain the breaking up of a meteor it has been suggested that the air
pressure acting on a surface which is probably quite irregular causes the
meteor to spin. The meteor is not strong enough to withstand the centrifugal
force generated by very rapid rotation and therefore breaks into fragments.
The name meteor was originally used for any phenomenon occurring in
the atmosphere. Thus the aqueous meteors included precipitation of all forms
as well as clouds, mist, etc. ; luminous meteors comprised aurora, rainbow,
halos and mirages. Only within the last 100 years has the meaning become
restricted to " shooting star."
Meteorograph.—A self-recording instrument which gives an automatic
record of two or more of the ordinary meteorological elements. Of late
the term has been more generally applied to the instruments that are attached
to kites or small balloons and sent up to ascertain the pressure, temperature
and humidity of the upper atmosphere. See BAROTHERMOGRAPH.
Meteorology.—The science of the atmosphere ; from the Greek /zerecopof,
lofty or elevated (to. /terecupa, atmospheric phenomena) and Aoyog, discourse.
Modern meteorology includes the study of the physical processes which occur
in the atmosphere, and of the connected processes of the lithosphere and
hydrosphere.
Metre.—The unit of length in the METRIC SYSTEM. It was intended to
be equal to one forty-millionth of the Paris meridian, but errors entered into
the calculation and it must now be considered as an arbitrary length. It is
the distance, at the melting point of ice, between two lines engraved upon a
platinum iridium bar kept in Paris and equals 1,553,164-1 wave-lengths of
the Cadmium red ray in dry air at 15° C. (H scale) and 760 mm. pressure.
1 metre = 10 decimetres = 100 centimetres = 1,000 millimetres.
The Order in Council of 1898 defines the inch as 25-400 millimetres, from
which it may be deduced that the metre is 39-370113 inches.
Micro-.—A prefix meaning small from the Greek fuxpog, for example,
microbarograph, microseism. Micro is sometimes used in the sense of one
millionth, for example, microfarad.
Microbarograph.—An instrument designed for recording small and rapid
variations of atmospheric pressure. It consists of an air-tight reservoir of
ample size containing air, and the difference of the external atmospheric
pressure and the internal pressure in the reservoir is made to leave a record
on a drum driven by clockwork. The reservoir is well protected from changes
of temperature by a thick covering of felt or other non-conducting material,
and it is also provided with a small leak, the magnitude of which can be
adjusted. If the external pressure changes slowly the leak allows the internal
pressure to follow it closely, but as the leak is small, the internal pressure
cannot adjust itself rapidly to any sudden changes in the external pressure, and
consequently a record of such changes is obtained.
Microclimate.—A modification of the general climate produced by the
immediate environment. Thus while the climate of an area such as the
county of Hertfordshire may be broadly described in general terms, important
differences are found over relatively short distances, due to ground contours,
geological factors, plant cover, etc. In general, valley bottoms experience a
larger diurnal range of temperature than the adjacent hills, together with an
intensification of phenomena such as frost and morning fog associated with
nocturnal radiation. Important differences are also found between the climates
of cities and those of adjacent rural districts. In ecological studies a distinction
135
MINIMUM
is sometimes drawn between the " local climate " (i.e. the climate of the
habitat) and the " microclimate " (i.e. the climate of the immediate sur­
roundings of the subject of study). In agriculture the term " microclimate "
has been applied to the meteorological conditions within the crop ; for
example, among the ears and stems in a cornfield.
Microseisms.—Seismographs are designed to record the tremors which
pass through the ground from distant earthquakes but these instruments
reveal also the fact that the ground is nearly always oscillating. The con­
tinuous oscillations are known as microseisms. The amplitude of microseismic
disturbances is conveniently expressed in terms of the unit denoted by fi;
this unit is the millionth part of a metre. In the British Isles the average
amplitude of the microseismic oscillations recorded by a seismograph varies
from about O-Sjx in July to about 2-6(jt in January. The average period is
about 6 seconds. The oscillations are always fluctuating, however. During
one half minute the amplitude may increase ; during the next half minute
the oscillations die out as if there were interference between waves from
different origins.
Microseisms are most vigorous at times when there are strong winds over
not too distant seas. The theory put forward by the German seismologist,
E. Wiechert, that microseisms are produced by the sea waves beating on steep
rocky coasts is supported by some observations but is not accepted by all
seismologists. An alternative explanation is that the variations in pressure
on the surface of the sea set up the microseismic oscillations.
Mile.—The statute mile is defined as 1,760 yds. or 5,280 ft. The geo­
graphical mile is the length of an arc of one minute of latitude, and varies with
latitude, being equal to (6076-8 — 31 • 1 cos 2({>) ft. or (1852-2—9-5 cos 2<j>) m.
in latitude <{>. The nautical mile, as used in hydrographic surveying is
identical with the geographical mile, but in general practice the nautical
mile is taken as 6,080 ft., the approximate value of the geographical mile in
lat. 50°.
Millibar.—The thousandth part of a BAR, which is the meteorological
unit of atmospheric pressure on the C.G.S. system. Since the bar is equal to
a pressure of one megadyne per square centimetre, i.e. to 1,000,000 dynes
per square centimetre, a millibar is equivalent to 1,000 dynes per square
centimetre.* The millibar has been in general use in the Meteorological
Office since May 1, 1914. The principal advantage of using a unit of this
type is that a statement of atmospheric pressure as a certain number of
millibars is perfectly definite. According to the older practice a separate unit
had to be used for length in reading the height of the mercury in the barometer,
generally the inch or the millimetre, but this length is not a measure of the
atmospheric pressure until the density of the mercury, the temperature of
the scale and the value of gravity at the place are allowed for. The " millibar "
on the other hand can only be used for pressure.
One thousand millibars are equivalent to the pressure of a column of
mercury, 750-1 mm. (29-531 in.) high at 0° C. (273°A.) in latitude 45°.
Minimum.—The lowest value reached by an element in a given period.
See EXTREMES.
* It should be explained here that a megadyne is a measure of force. The dyne is the
unit force of the C.G.S. system of units, and stands for the force which produces unit
acceleration in one gram. As the force of gravity is the most familiarly known of all forces,
we may say that the force of one dyne differs but little from the weight of a milligram,
and a megadyne stands in the same relation to the weight of a kilogram. The precise
numerical relation is dependent upon locality, because the weight of a body, that is, the
force which gravity exerts on it, depends upon latitude and the distance from the earth's
centre. At sea level, in latitude 45°, the gram weighs 980-6 dynes, the kilogram 0-9806
megadynes.
MIRAGE
136
Mirage.—The term mirage is used for certain appearances produced by
the refraction of light by the atmosphere ; these appearances include the
illusion of a sheet of water in the desert as well as cases in which definite
distant objects can be seen in duplicate.
Over a heated desert the air very close to the ground is less dense than
that above it so that the velocity of light is greatest close to the ground.
Rays coming down from the sky at a gentle inclination may be bent up
again to the observer, to whom the rays appear to come from a bright water
surface. The banks, reeds, etc., are the images of various objects distorted
by being viewed through layers of air of varying density so that a dark stone
appears as though it were an upright stake. Hills situated a short distance
away may appear as detached masses floating on this lake-like surface, their
lower portions being invisible under the conditions prevailing. Mirages may
often be seen over smooth road surfaces on calm hot days in England, especially
over tarred roads.
These are all cases of inferior mirage. " Superior " mirage in which the
light rays are bent downwards from a warm stratum of air resting on a
colder is seen most frequently in polar regions. The distances involved are
greater so that the details can hardly be observed without telescopic aid.
With such assistance a distant ship may sometimes be seen in triplicate,
one of the images being inverted. As the stratification which produces
superior mirage is stable, the images are clear and well defined in contrast
to the shimmering images of the inferior mirage.
The images produced by superior mirage are not always exactly in the
same vertical. The separation is explained by a slight departure of the strata
of equal density from horizontal planes. Lateral refraction of this kind is to
be distinguished from the phenomenon which can be observed sometimes
near a heated wall when .objects appear to be reflected in the wall.
Mist.—See FOG.
Mistral.—A north-westerly or northerly wind which blows off-shore along
the north coast of the Mediterranean from the Ebro to Genoa. In the region
of its chief development its characteristics are its frequency, its strength, and
dry coldness. It is most intense on the coasts of Languedoc and Provence
especially in and off the Rhone delta. On the coast the speeds experienced
are about 45 mi./hr. but in the Rh6ne valley over 85 mi./hr. has been reached.
Mock Moon.—See PARASELENAE.
Mock Sun.—See PARHELION.
Mock Sun Ring or Parhelic Circle.—The circle passing through the sun
parallel to the horizon. The whole of this circle may be bright but colourless.
The phenomenon is explained by the presence of ice crystals with vertical
facets from which the light is reflected. See HALO PHENOMENA.
Mode.—In a series of values, the value of most frequent occurrence. An
approximate rule, when there is only one mode, is
mode = median — 3(mean — median).
Monsoon.—The name is derived from an Arabic word for " season,"
and originally referred to the winds of the Arabian sea, which blow for
about six months from the NE., and six months from the SW. It has been
extended to include certain other winds which blow with great persistence
and regularity at definite seasons of the year. The primary cause of these
winds is the seasonal difference of temperature between land and sea areas.
The winds are analogous to land and sea breezes, but their period is a year,
instead of a day, and they blow over vast areas, instead of over a limited region.
Near the equator the seasonal changes of temperature are in general
too small to cause the development of monsoons ; in the higher latitudes
of the prevailing westerlies and in the polar regions the wind components
due to contrast of temperature between land and sea are at most sufficient
only to modify slightly the " planetary " wind circulation, so that the regions
most favourable to the development of monsoon conditions are in middle
latitudes, near the tropics.
137
NEPHOSCOPE
The moisture content (determined by the length of sea track pursued)
of the landward-blowing winds and the topography of the land on to which
the moisture-carrying winds blow have great influence on the amount of
rainfall associated with the landward monsoon. Owing to a suitable
combination of the various factors monsoon conditions reach their greatest
development over eastern and southern Asia, in most of which regions a
rainy summer monsoon from the SW. is the outstanding feature of the climate.
In the countries of its occurrence the term " the monsoon " is popularly used
to denote the rains, without reference to the winds.
Monsoon conditions occur also, but to a much less extent in north
Australia, parts of western, southern and eastern Africa, and parts of North
America and Chile.
Month.—See LUNAR, CALENDAR.
Moon.—The only satellite revolving round the earth. Its sidereal period
of revolution (i.e. with respect to the so-called " fixed " stars) averages about
27J days, but varies as much as three hours from this value on account of the
eccentricity of its orbit, and its " perturbations." The synodic period of
revolution (i.e. the interval from new moon to new moon) has a mean value
of about 29 • 53 days, but varies about 13 hours on account of the eccentricities
of the orbits of the moon and the earth.
The brilliance of the moon is due solely to sunlight falling upon it. The
fallacy that the moon's rays are injurious to plants arises from the fact that
nights of ground frost are clear nights, on which the moon will be visible if
suitably placed.
Moon, Phases of.—The appearance of the moon, by custom restricted
to the particular phases of new moon when nothing is visible, first quarter
when a semicircle is visible with the bow on the west, full moon when a full
circle is visible, and last quarter when a semicircle is visible with the bow
on the east. The changes of phase are due to the changes in relative position
of earth, moon and sun. The moon rotates on its axis once in each orbital
revolution, and so the same face of the moon is always turned towards the
earth. The popular belief in the association of weather changes with the phases
of the moon is not confirmed by meteorological investigations, though
S. Chapman* has shown that there is a very minute lunar tide in the atmosphere,
giving a variation of pressure of only a small fraction of a millibar.
Mother of Pearl Clouds.—These were first noted and so named by Mohn ;
they were seen by Stormer in 1890 and seen and photographed in Scandinavia
in 1932. Their somewhat rare appearances have been during winter months
on occasions when an extensive and deep depression lies over northern
Scandinavia. The clouds are somewhat lenticular in form, very delicate in
structure, and show very brilliant irisation at angular distances up to 40°
from the sun's position. The colouring endures for some considerable time
after sunset. The mean height of these clouds according to Stormer's
calculations is 24 Km. and thus they lie well within the stratosphere; the
diameter of the cloud particles has been computed as not exceeding 0 • 0025 mm.
Mountain Breeze.—A breeze which blows at night down valleys and
mountain slopes. At night the mountain slopes become cooled by radiation,
the air in contact with them is also cooled and its consequent increase in
density causes it to flow downwards. See KATABATIC.
Nephoscope.—An instrument for determining the direction of motion
of a cloud and its angular velocity about a point on the ground vertically
below it. The types most commonly employed are the Fineman reflecting
nephoscope and the Besson comb nephoscope, descriptions of which will be
found in the " Meteorological Observer's Handbook." A camera obscura
arranged to project a view of the clouds near the zenith on to a graduated
board may be used as a nephoscope.
• London, Proc. roy. Soc., A 151, 1935, pp. 105-17.
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NEUTRAL POINT
138
Neutral Point.—See POLARIZATION.
Night Sky, Light of.—See AURORA PERMANENT.
Nimbus (Nb.).—A term formerly used for a dense layer of dark shapeless
cloud with ragged edges from which steady rain or snow usually falls. In
1933 this name was combined with other names of clouds, e.g. nimbostratus,
fractonimbus and is now no longer used alone.
Nimbostratus (Ns.).— A low, amorphous and rainy layer, of a dark grey
colour and nearly uniform ; feebly illuminated seemingly from inside. When
it gives precipitation it is in the form of continuous rain or snow.
But precipitation alone is not a sufficient criterion to distinguish the cloud
which should be called nimbostratus even when no rain or snow falls from it.
There is often precipitation which does not reach the ground ; in this case
the base of the cloud is always diffuse and looks "wet " on account of the general
trailing precipitation, virga, so that it is not possible to determine the limit of its
lower surface.
Noctilucent Clouds.—These have been seen several tunes in both hemispheres
and always in the summer months at the place of observation and during the
midnight hours. They appear most frequently just after the solstices. In
appearance they resemble cirrus but have a bluish-white to yellowish colour.
Photogrammetric measurements assign to them a mean height of 82 Km.
They usually move from north-east with velocities of from 100 to 300 miles
per hour, but may also move from other directions at lower speeds. Nothing
is known as to their composition.
Normal.—The name given to the average value over a period of years
of any meteorological element such as pressure, temperature, rainfall or
duration of sunshine. Owing to the fact that the average of any finite number
of observations of a variable quantity is subject to a " standard error "
depending upon the variability of the observations, it follows that the normals
available to meteorologists must be subject to uncertainty. The uncertainty
can be reduced by increasing the number of observations, and it is an advantage
therefore to use as long a period as possible in calculating meteorological
normals, provided the observations are known to be homogeneous. The
degree of uncertainty associated with averages for relatively short periods
may be illustrated by the following data for Oxford (Radcliffe Observatory).
x
Period
„
„
„
„
1921-30 ..
1881-1915
1901-30 ..
1815-1935
1
1
•—1>
10 years, 1921-30 .. 2-55
1881-1915 1-81
35
1901-30 .. 2-16
30
1815-1935 1-97
121
10
35
30
121
February
1-90
1-64
1-71
1-63
I
<
&
&
I
1—1
_>>
"3
i—)
*•
P
1
<
September
October
November
December
i*
cd
o>
><
(a) Rainfall (in.)
1-28 1-97 1-90 1-36 2-50 2-41 2-36 2-40 2-89 2-69 26-21
1-65 1-60 1-87 2-24 2-37 2-28 1-71 2-89 2-30 2-46 24-82
1-88 1-93 1-98 2-03 2-52 2-34 1-86 2-74 2-28 2-78 26-21
1-59 1-78 1-94 2-17 2-45 2-38 2-39 2-73 2-30 2-20 25-53
(b) Temperature (° F.)
40-7 39-7 42-7 46-0 52-9 57-7 62-2 60-1 56-4 50-2 42-5 40-3 49-3
38-4 39-7 42-1 47-0 52-8 58-5 61-9 60-9 56-6 49-4 43-5 39-9 49-2
40-0 40-2 42-9 46-8 53-7 57-9 61-8 60-8 56-7 50-5 43-1 40-7 49-6
38-4 39-5 41-9 46-5 52-6 58-5 61-3 60-5 56-1 49-5 43-1 39-9 49-0
In practice it is difficult to maintain complete uniformity in the conditions
of observation over long periods. Also, it is clearly desirable for purposes of
comparing the climates of different places that the adopted normals should
refer to similar periods. For these reasons it is the practice to select a standard
period for the computation of normals for routine use, notwithstanding the
fact that at certain stations the observations have extended over longer
139
OCCLUSION
periods. In the British Isles the present standard period for rainfall normals
is the 35 years 1881-1915. For sunshine and temperature, normals are
calculated for 30-year periods and are revised every five years. The period
1901-30 was adopted by the International Meteorological Organization at
Warsaw in 1935 as a standard period for climatological normals.
Charts of the normal distribution of various climatic elements are available
for many areas in the form of climatological atlases. For the British Isles
monthly and annual rainfall charts are given in the " Rainfall Atlas of the
British Isles " published by the Royal Meteorological Society, and charts
of other elements are to be found in the " Book of Normals " (Section III),
"Averages of Humidity" and in E. G. Bilham's "Climate of the British Isles"
(London, Macmillan, 1938). World charts for various elements are included
in Shaw and Austin's " Manual of Meteorology " Vol. 1 1, Cambridge, 1936.
Normal Law (of Errors).—When the error to which an observation is
subject may be regarded as made up of a large number of small independent
errors, each as likely to be positive as negative, it can be shown that the
probability that one observation should yield a result having an erroi between
x and x + dx is
h
where A is a parameter. If a large number n of observations be made we
should therefore expect a number
nh
^ e~h**Z dx.
to lie between the limits x and x + dx. The observations are then said to
satisfy the " normal law of errors." It will be noted that the FREQUENCY
is the same for + x as for — x, so that the frequency curve is symmetrical
about the axis of y. Many proofs of the normal law are to be found in text
books on " least squares," but no proof is altogether satisfactory. Further
it must be noted that distributions of observations are frequently met,
especially hi biological and allied sciences, which are not symmetrical.
The parameter h is related to the STANDARD DEVIATION a by ha— -7071.
Norte, Norther. —A strong, cold, northerly wind which blows mainly in
winter on the shores of the Gulf of Mexico. Here it is sometimes humid and
accompanied by precipitation. The northers reach the Gulf of Tehuan tepee
as cold, dry winds, where they often set in suddenly and quickly raise a heavy
sea.
Observer.— In meteorology, a person who undertakes, in co-operation
with others at a r6seau of stations, to make regular and simultaneous records
of the weather upon an organised plan. Good observing requires punctuality
and accuracy, and therefore skill in reading and setting instruments, and
intelligence in noting occurrences which are worth recording, though they are
not in the prescribed routine. The best observer is one who is personally
interested in scientific work on a co-operative basis.
Occlusion.—A newly developed DEPRESSION has in accordance with the
Bjerknes theory a WARM SECTOR running out from the centre which contains
warm or tropical air, the rest of the depression at the earth's surface being
occupied by cold or polar air. The front of the warm sector is the WARM FRONT
of the depression while the back of it is the COLD FRONT, these fronts marking
the chief regions of cloud and rain in the depression. Owing chiefly to the
inertia of the cold ak ahead of the WARM FRONT the cold front closes on the
warm front. The warm sector is then reduced to a line called the line of
occlusion and subsequently is lifted from the surface of the earth. It is still
present, however, in the upper layers and the cloud and rain associated with
L 90565)
E*2
OCEAN CURRENT
140
the fronts are therefore to be found along a line of occlusion for some time
after the warm air has left the earth's surface. There is also often a difference
between the two cold air masses which enables the occlusion to persist as a
front. The majority of depressions which reach Europe from the Atlantic
are occluded by the time they reach the seaboard, and lines of occlusion are
thus much more frequent upon European weather charts than warm and cold
fronts.
Ocean Current.—See CURRENT, OCEAN.
Okta.—Unit, equal to area of one eighth of the sky, used in specifying
cloud amount.
Ombrometer.—Another name for RAIN-GAUGE.
Optical Phenomena.—The optical phenomena of interest to meteorologists
are numerous. They include the BLUE OF THE SKY, SUNRISE AND SUNSET
COLOURS, the production of RAINBOWS, CORONAE and HALOS, the fading of
daylight and the twinkling of the stars. These matters are very fully dealt
with in the great treatise of the Austrian meteorologist Pernter, the second
edition of which was prepared by his pupil and collaborator F. M. Exner.*
The best account in English is to be found in Humphreys's " Physics of the
Air."t
Orientation.—From Oriens (Lat.), the rising of the sun—the east. The
direction of an object referred to the points of the compass.
Orographic Bain.—Rain caused by the interference of rising land in the
path of moisture-laden wind. A horizontal air current striking a mountain
slope is deflected upwards, and the consequent dynamical cooling associated
with the expansion of the air produces rain if the air contains sufficient
aqueous vapour. Mountain ranges at right angles to the direction of the
wind offer greater obstruction to the wind and therefore are more likely to
produce rain than mountain ranges which lie in the direction of the wind.
In the case of isolated mountain peaks, the wind is often able to go round
and the amount of rain produced may be trifling. The prevailing wind of
these Islands is from the SW. and as this damp wind passes across the country
the aqueous content decreases. A mountain range of a given height along the
west coast would therefore produce a greater rainfall than a range of similar
height and position relative to the wind in the east. Orographic rains are
typically persistent and usually widespread. Their great importance lies in
the fact that the main controlling factor, the rising ground, is always operative,
and naturally, always in the same place. Thus a station on a hill is not only
usually wetter year by year than a neighbouring station in a valley, but the
proportion remains fairly constant. Some valleys, if they are surrounded by
mountains on all sides, receive nearly as large a rainfall as the surrounding
mountains since air is generally rising as it passes over these valleys. See
CONVECTIONAL RAIN, CYCLONIC RAIN.
Oscillation.—A periodic movement to and fro, or up and down, or a
periodic variation of a quantity above and below its mean value. The simplest
dynamical illustration is the simple pendulum, which swings first to one
side of the vertical then to the other. The term oscillation is used in
meteorology in connexion with a wide variety of phenomena, such as the
diurnal variation of pressure, the annual variation of temperature, the gusts
and lulls of wind, etc.
Overcast Day.—Since January 1, 1949 defined in Great Britain as a day
on which the average CLOUDINESS at the hours of observation is more than
6 OKTAS of the sky. Before 1949 it was defined as a day on which the average
CLOUDINESS at the hours of observation was more than 8 tenths of the skyf.
* Pemter, J. M., and Exner F. M.
" Meteorologische Optik."
Wien and Leipzig,
t 3rd Edition, New York, 1940.
j Codex of Resolutions adopted at International Meteorological Meetings, 1872-1907,
London, 1909.
141
PARALLAX
Ozone.—An allotropic form of oxygen for which the chemical symbol
is O3. It is produced by passing electrical sparks through oxygen or by the
action of cathode or ultra-violet rays. The amount of ozone in the atmosphere
is best determined by spectroscopic observations, as ozone absorbs ultra-violet
light of certain wave-lengths. By this test it can be shown that there is very
little ozone in the atmosphere near the ground. There is, however, a region
in the upper atmosphere where ozone is comparatively plentiful.* The
quantity of ozone can be determined by the analysis of sunlight, and is found
to be such that if it could be brought down to the ground it would form a
layer about 3 mm. thick. The most reliable method of determining the height
at which the ozone occurs is by examination of the ultra-violet light from the
zenith sky at different times of day. The bottom of the region in which ozone
occurs is about 20 Km. above the ground. The amount shows considerable
variations. The zone absorbs ultra-violet light coming from the sun and it is
believed that the energy derived from this ultra-violet light maintains a very
high temperature in the upper atmosphere. See METEORS and AUDIBILITY.
Ozone, Measurement of.—Measurements of absorption by ozone have been
made by means of a spectrograph incorporating a quartz Fery prism and an
optical wedge, scattered light of wave-lengths outside the region used being
absorbed by a mixture of the chlorine and bromine vapourf. An instrument
employing a photo-electric cell and amplifier has also been designed, by which
a direct comparison of the intensity of two wave-lengths in sunlight or in
skylight can be made.J
Pack Ice.—Sea ice which has drifted from its original position. It is
termed " close pack " if the floes are mainly in contact and " open pack "
if the floes for the most part do not touch.
Pallium.—Occasionally used in the past for NIMBOSTRATUS.
Pampero.—A name given in the Argentine and Uruguay to a severe
storm of wind, sometimes accompanied by rain, thunder and lightning. It
is a LINE-SQUALL, with the typical arched cloud along its front. It heralds
a cool south-westerly wind in the rear of a DEPRESSION ; there is a great drop
of temperature as the storm passes.
Parallax is an apparent change in the position of an object caused by a
change in the position of the observer. In connexion with the reading of
meteorological instruments an error of parallax may arise whenever the
indicator of the instrument (e.g. end of column of mercury or water, pointer,
etc.), and the scale against which the indicator is to be read are at a distance
from one another which is comparable with the length of the smallest readable
scale division ; for in such a case a movement of the observer's head may
cause his line of vision to the indicator, to intersect the scale at different
points and so give rise to different readings. The error is eliminated by
ensuring that the line of vision to the indicator is at right angles to the scale
when the reading is made.
Parallax is unfortunately almost universal among meteorological instru­
ments. Thus the distance of the scale of the ordinary Kew-pattern barometer
from the top of the mercury meniscus is about half an inch, and as an accuracy
of -001 in. in the scale reading is usually required, the error of parallax may
in this case be very serious. Barometers are, however, designed to ensure
that errors of parallax can be completely eliminated when reading. An error
of parallax of £° F. may easily be made in reading a thermometer in a
Stevenson screen ; to avoid such errors, care must be taken to see that
imaginary lines drawn towards the observer at right angles to the scale at
all points, at which readings may be taken, are not obstructed by part oi
* London, Quart. J.R. met. Soc., 51, 1925, p. 367 and London, Proc. toy. Soc., 120, 1928.
p. 255.
t London, Quart. J.R. met. Soc., 51, 1925, p. 367 and London, Proc. roy. Soc., 110, 1926
pp. 660-93, and 120, 1928, p. 255.
f London, Proc. Phys. Soc., 43, 1931, pp. 324-39.
PARAMETER
142
the screen itself, or by other thermometers placed in the screen. Errors of
parallax in reading a rain measure are also important, and they occur
whenever an observer places his eye either above or below the level of the
water surface in the rain measure.
Parameter.—A quantity related to one or more variables in such a way
that it remains constant for any specified set of values of the variable or
variables, e.g. in STATISTICS, MEAN, STANDARD DEVIATION.
Paranthelia.—A mock sun at the same elevation as the sun and in an
azimuth greater than 90° from the sun may be called a paranthelion. White
paranthelia at 120° from the sun are fairly common. Paranthelia at about
140° from the sun have been recorded on rare occasions.
Paraselenae or mock moons, analogous to mock suns, have been observed
occasionally. No measurements are available but it may be presumed that
mock moons will be at the same elevation as the moon and that the angular
distance will vary like that between sun and mock sun, being 22° when the
luminary is on the horizon and 25° when the luminary is at an elevation of 30°.
Parhelia.—Images of the sun, coloured or white. The mock suns seen
most frequently are at the same elevation as the sun and coloured with
red nearest the sun. When the sun is near the horizon the distance is equal
to the radius of the ordinary halo, i.e. 22°. When the sun is higher the
distance is greater so that if halo and mock sun are both seen the mock
sun is outside the halo. White mock suns are to be seen in the azimuth
120° from the sun. Bright patches seen at the top and bottom of the halo
of 22° at the points of contact of the tangent arcs, are sometimes referred
to as mock suns
Pentad.—A period of five days. Five-day means are frequently used in
meteorological work, as five days form an exact sub division (73rd) of the
ordinary year, an advantage not possessed by the week.
Percolation.—The downward passage of surface water through the soil.
Part of the rain which falls on the land surface re-evaporates, part runs off
into streams and rivers to the sea, while part percolates through the soil.
Measurements of the amount of rain water which percolates through certain
depths of soil have been published in the annual volumes of British Rainfall.
Usually the gauge consists of a cubic yard of natural earth inserted in a
metal container and sunk in the hole formed by removing this earth. The
rain water which percolates through is drained off and measured daily at
9h., access to the receiver being obtained by means of a trap door at the
side of the gauge. The results are usually published as a depth in hundredths
or thousandths of an inch of water. See EVAPORATION.
Periodical.—Recurring at regular intervals, like the position of the bob
of a simple pendulum. The most obvious periodical variations in the atmos­
phere are associated with the alternation of night and day, and the alternation
of winter and summer, corresponding respectively with the day and the
year. Strictly speaking a variation is not to be called periodical unless the
interval between successive maxima and successive minima is constant, and
so the sunspot variation should not strictly be described as periodical, since
the interval between successive maxima or minima varies from about 8 years
to about 16 years.
Periodicity.—A periodical variation. A vast amount of labour has been
devoted by meteorologists to the search for periodical variations other than
those whose periods are the day or the year, by the use, at some stage or
other of the work, of the methods of HARMONIC ANALYSIS. It is thus implied
that a periodicity as normally treated, shall be at least approximately of
the nature of a harmonic oscillation, capable of being represented by a simple
sine curve. A periodicity requires for its complete determination the length
of the period, the amplitude (i.e. half the total range) of the variation, and
143
PHENOLOGY
the time of occurrence of the maximum. If a periodicity is to be of value
in forecasting, the square of its amplitude must be an appreciable fraction
of the square of the STANDARD DEVIATION of the original observations.
Periodogram.—A diagram used in a method devised by Schuster for the
investigation of hidden periodicities. In meteorological phenomena, the
variations from day to day or from month to month are so irregular that
usually the existence of any period other than the yearly one is masked.
Schuster's method of finding the lengths and amplitudes of periods which
are thus masked by apparently casual variations consists in taking different
trial periods T, and evaluating the amplitude R for each trial period. A graph
is drawn with Rz as ordinate and T as abscissa. Usually the diagram thus
obtained shows a number of peaks which stand out above the general level
of the curve. The values of T corresponding with these peaks are taken
to be the most likely periods. There is no special advantage in graphing
Rz and T rather than R and T, since the same peaks stand out in the graph
of R and T. If the original n observations formed a random distribution,
with standard deviation a, the expectation (or mean value) of R2 would be
4a2/(n — 1). Schuster has shown that the probability that any particular value
of R2 should exceed « times 4ozj(n — 1) is e~". This expression is frequently
used to test the reality of periods revealed by the periodogram, but the process
requires considerable care.
Persistence.—In meteorology, the tendency for abnormal conditions of the
same type to continue over a longer period than usual; thus we speak of a
persistent anticyclone. E. V, Newnham* has shown that the tendency for
the persistence of wet or fine weather increases with the length of the period
through which similar conditions have already persisted. Thus at Greenwich
after one day without rain, the probability that the next day will be rainless
is -57, but after ten successive fine days, the probability has risen to -80.
Similarly, after one rain-day, the probability of a rain-day is -59, but after
ten successive rain-days it is -70.
Personal Equation.—An expression used to denote the correction which
might or should be applied to an observer's readings of an instrument, in
consequence of an unconscious tendency on his part to read too high or too
low. The tendency is usually nearly constant for any given observer reading
a given instrument. A familiar example is that of reading the rain-measure.
The observer is directed to hold the measure so that the meniscus is level
with the eye, but it is difficult for an observer to judge when this condition
is correctly secured. Some observers hold the glass always too high, resulting
in readings which are too low, others possess the opposite tendency.
Phenology.—The study of the sequence of seasonal changes in nature.
All natural phenomena are included, seed-time, harvest, flowering, ripening,
migration, and so on, but often in practice the observations are limited to
the time at which certain trees and flowering plants come into leaf and flower
each year, and to the dates of the first and last appearances of birds and
insects.
A phenological report is published each year by the Royal Meteorological
Society, in which the phenological data contributed by observers stationed
all over the British Isles are brought into relation with the weather of the
year under review.
A " bioclimatic law " has been proposed by Dr. Andrew Hopkins of the
United States Weather Bureau. It was determined from phenological obser­
vations made in the eastern United States, and states that a progression or
regression (according to season) of plant life occurs there, which is at the
rate of four days for each degree in latitude or five degrees of longitude, or for
400 ft. of altitude. The law appears to hold approximately for the British
Isles in so far as changes of latitude and altitude are concerned.
* " The persistence of wet and dry weather."
p. 153.
London, Quart. J.R. met. Soc., 42, 1916,
PHENOMENON
144
Phenomenon.—" That ot which the mind or senses directly takes note."
The word is generally used in meteorology, however, to denote either (1) an
unusual intensity of some occurrence, e.g. " ugly " sky, high rainfall, low
temperature, high pressure, gale; or (2) occurrences which are only
occasionally noted, e.g. thunder, halo, fog, glazed frost.
Phosphorescence.—The phosphorescence of the sea is one of the most
remarkable of natural phenomena and various explanations of it have been
attempted in the past. One theory held that sea water absorbed sunlight
by day and emitted it by night. In 1749-50, however, a small animalcule
giving a blue light was discovered by Vianuelli and Guixellani in the
Mediterranean and subsequent investigation has shown that animal life is
the origin of marine phosphorescence. Light production is due to a great
variety of organisms, from microscopic ones up to many forms of deep sea
fish, and the peculiarity of the light is that it is generated without heat, a
process never yet achieved in the laboratory. Different types of light have
been analysed and the colour is also found to vary through white, silver,
green blue and lilac to red. The method of production has been shown to
be some form of slow oxidisation which is entirely automatic in the lower
forms of life but is in some measure under control in the higher forms.
Recent research has shown that while phosphorescence may occur anywhere
it is most frequent in the warmer tropical seas and in particular in the
Arabian Sea, where it exhibits a definite maximum in August. In the North
Atlantic ocean some regions show summer maxima, while others have spring
maxima. It also appears that phosphorescence is more frequent in coastal
regions than in mid-ocean. One remarkable type of phosphorescence is the
diffused " milky sea " which may give light enough to read by and to
illuminate clouds. This type has often been reported as exerting a calming
effect upon the sea surface. More elaborate phenomena occur which have
not yet been satisfactorily explained, such as phosphorescent bands and the
great rotating phosphorescent wheels.
Photographs, Meteorological. —Whenever unusual meteorological phe­
nomena occur an attempt should be made to photograph them, if possible.
Notes accompanying photographs should give the date, time and geographical
position of the occurrence of the phenomenon, the direction from which the
photograph is taken, and the weather conditions prevailing at the time and
the relative sizes of the photograph and the original or the focal length of
the lens.
For photographing clouds, Cave* finds that panchromatic plates and
colour screens give the most satisfactory results ; he advises a moderately
deep yellow screen for all clouds except cirrus, and for cirrus a red screen.
For persons unfamiliar with panchromatic plates and colour screens, Clarkef
recommends very slow plates and hydroquinone developer.
Pilot Balloon.—A small free rubber balloon, filled with hydrogen for
obtaining the direction and velocity of the upper wind. The balloons
commonly employed have circumferences 40, 70, 90 and 150 in. when fully
inflated, and are usually specified by reference to these circumferences.
They are normally filled with hydrogen so as to have a known theoretical
rate of ascent, based upon the following formula :—
V = q L*I(L + W)*
where V is the upward velocity, W the weight of the balloon (and any
attachment), and L the free lift. If L and W are expressed in grams and
V in feet per minute, q is about 275. If V is in metres per minute the value
of q is 84 (83 • 8). Assuming a known rate of ascent, the balloon may be kept
in sight by means of a single theodolite of special construction, and the
direction and velocity of the wind in the interval between two readings of
* The forms of clouds. London, Quart. J.R. met. Soc., 43, 1917, pp. 61-82.
t Instructions for the taking of photographs of clouds. Paris, Commission Internationale
des Nuages et Office National Meteorologique de France.
145
POLAR AIR
the theodolite computed, using either graphical methods or a special pilotballoon slide-rule. At high altitudes owing to the balloon developing minute
holes and becoming porous, the balloon no longer rises at the assumed rate
and errors occur in this method of observation.
A more accurate method of observation involves the use of two theodolites,
one at each end of a measured base line, and the attendance of several
observers. By observing the balloon through the two theodolites the actual
height of the balloon can be calculated in addition to other factors. A third
method of observation is to attach a tail of known length to the balloon,
and measure the angle subtended between the end of the tail and the centre
of the balloon, by means of a graticule or divided scale in the eye-piece of
the theodolite. This method is useful, but occasionally a swinging tail is apt
to lead to incorrect values. For use at night an ordinary balloon is employed
with a light attached. Commonly, the light consists of a small Chinese
lantern., but in some countries an electric bulb lit by a small dry battery is
used.
Pitot Tube.—An instrument for determining the velocity of a stream of
fluid by measuring the increase of pressure above the " static " or undisturbed
pressure, in an open tube facing the stream. The velocity is computed from
the relationship p = £ p v z (where p is pressure, p density and v velocity).
Suitably mounted, a pitot tube may be used as an ANEMOMETER.
Pleion.—A term introduced by H. Arctowski to signify an area over
which some meteorological element, for example, temperature, is above
normal. Areas where the element is below normal are termed antipleions.
Arctowski drew his pleions by constructing overlapping twelve-monthly
departures from average, and he found a tendency for the pleions and anti­
pleions obtained in this way to persist for a considerable time, moving slowly
across the country.
Pluviograph.—A self-recording RAIN-GAUGE ; the rise of the water in the
gauge is recorded by means of a pen attached to a float. Some form of device
by which the gauge automatically empties itself when the water reaches a
certain height is often employed. See HYETOGRAPH.
Pluviometer.—A RAIN-GAUGE.
Pocky Clouds.—See MAMMATUS.
Polar Air.—Air originating in high latitudes. Air which arrives directly
from the ice-bound areas of the arctic is now usually known as ARCTIC
AIR. Polar air in present-day synoptic meteorology has usually an adjective
attached to it, most frequently " maritime," occasionally " continental."
Maritime polar air is often (in winter usually) of arctic origin, but it may
have originated over cold parts of the oceans, and its general feature (so far
as the British Isles are concerned) is that the surface layers have gained heat
from those parts of the Atlantic which are kept warm by the Gulf Stream.
Its surface temperature is generally below that of the sea, and its LAPSERATE of temperature is high at least for some thousands of feet. In both
arctic and maritime polar air INSTABILITY SHOWERS are frequent, and there
is sometimes thunder (chiefly near the coasts in winter and over land in
summer), but this requires cold air to an adequate height, which exists
most frequently in depressions, at a sufficient distance from the warm sector
(if any). Polar air has a marked tendency to subside and this results in
stability, except sometimes in the lowest few thousand feet. See SUBSIDENCE,
AIR MASS.
Sometimes maritime polar air travels far to south and then returns, and
so begins to lose heat at the surface. This is the only type of polar air which
has a high dew point.
Continental polar air is very cold in the winter, but in summer it gives
sunny and comparatively warm weather.
Currents consisting of polar air are called polar currents.
POLAR FRONT
146
Polar Front.—The line of discontinuity, which is developed in suitable
conditions between air originating in polar regions and air from low latitudes,
on which the majority of the DEPRESSIONS of temperate latitudes develop.
It can sometimes be traced as a continuous wavy line thousands of miles
in length, but it is interrupted when polar air breaks through to feed the
trade winds, and is often replaced by a very complex series of FRONTS, or by
continuous gradients of temperature.
Polarimeter.—A polariscope provided with circles or other equipment for
making quantitative observations upon the state of polarization.
Polarization.—Waves may be made to run along a taut rope by shaking
one end. The wave motion may be confined to one plane ; or it may be chiefly
in one plane ; or it may take place equally in all directions round the rope.
Similarly, the waves which run along a ray of light must belong to one of
these classes and the ray is accordingly said to be completely polarized,
partially polarized or unpolarized as the case may be. The plane of polarization
is defined as the plane in which the wave motion is a minimum, or, in terms
of the electromagnetic theory of light, the plane in which the electric vector
is a minimum.
Light reflected from the surface of water or glass is (in general) partially
polarized in the plane containing the incident and reflected parts of the ray.
That is to say, the excess motion occurs parallel to the surface of the water
or the glass. Light coming from the blue sky is also partially polarized.
This discovery was made by Arago in 1809. The polarization is most marked
in the light which comes from a region in the solar vertical at 90° from the
sun. In this case some two-thirds of the wave motion is confined to the
plane containing the sun, the observed point and the observer; or, in terms
of the electromagnetic theory, the electric vector in the wave front is at right
angles to this plane. In or near to the solar vertical are three neutral points,
or small regions, the light from which is unpolarized. They are named after
Arago, Babinet, and Brewster, and their approximate positions are respec­
tively, 160° from the sun (i.e. 20° above the anti-solar point), 20° above and
20° below the sun. So-called neutral lines pass through points in which the
plane of polarization is inclined at 45° to the vertical and also through the
neutral points mentioned. When the sun's altitude exceeds 20° the plane of
polarization of light received from regions (excluding the vicinity of neutral
points) not within 30° of the sun does not usually differ greatly from the plane
containing the sun, observer and observed point. The degree of polarization
and the position of the neutral points depend on the sun's altitude, on the
wave-length of the light examined, on the degree of turbidity of the atmosphere,
and in consequence of the last factor, on the weather conditions. Factors
promoting an increase of the amount of light reflected through the atmosphere
(e.g. a layer of snow on the ground) decrease the percentage of polarization.
Light from the clear sky at night is only very feebly polarized.
The phenomena of polarization of sky light are to be attributed to the
scattering of light. It was demonstrated theoretically by Rayleigh (third
Baron) that the light scattered, by air molecules or other particles small in
comparison with the wave-length of light, in a direction perpendicular to
the sun's rays should be completely polarized in a plane containing the
incident and scattered rays ; polarization should be less complete in other
directions and non-existent in either direction along the incident rays.
Rayleigh (fourth Baron) and others have shown experimentally that polarized
light is scattered from a beam of ordinary light traversing dust-free air;
polarization is in the sense indicated by theory and is a maximum, about
96 per cent., in a direction perpendicular to the primary beam. Although
theory and experiment thus account satisfactorily for the orientation of the
plane of polarization of the sky light, they fail to explain the degree of the
polarization, which is found to be much less than we should expect. Moreover,
the occurrence of neutral points and other phenomena remains unaccounted
147
PRECIPITATION
for. A point of the sky is illuminated not only by the sun but also by light
from other parts of the sky. According to Soret, Ahlgrimm, and others, the
light due to secondary scattering by the air molecules and small suspended
particles is partly unpolarized and partly polarized in the horizontal plane,
i.e., perpendicularly to the polarization due to primary scattering. This theory,
which is not universally accepted, indicates why there is incomplete polariza­
tion even in the skylight received perpendicularly to the sun's rays ; and
-also that the two polarization effects cancel to produce unpolarized light at
certain points, the neutral points, on the sun's vertical.
Pollution.— See ATMOSPHERIC POLLUTION.
Polymeter (Lambrecht's).—An instrument combining a thermometer and
a direct reading hair hygrometer. The hairs, which hang vertically, operate
a pointer which indicates directly the relative humidity and the depression
of the dew-point below the air temperature. The thermometer gives the air
temperature and the dew-point can thus be readily ascertained from the
readings of the two instruments.
Ponente.—A westerly wind which blows in the Mediterranean.
Poorga (Purga). — See BURAN.
Potential as applied to energy indicates the ENERGY which is due to the
position of a body. In considering the total amount of energy available in
any case we must consider not only the position but the quantity of working
substance that is collected there. If we wish to consider the influence of the
position alone we must limit our ideas to a particular amount of the working
substance. We naturally choose the unit measure as the amount for this
purpose, and the potential energy of unit quantity is called the potential at the
point. Thus, the electrical potential at any point in the atmosphere is the
-amount of energy which one unit of electricity possesses in virtue of its
position at the point. Similarly, the gravitational potential or GEOPOTENTIAL
at any point above the earth's surface is the potential energy of a unit quantity
of material, a gram or a pound, placed there.
Potential Temperature.—The temperature which a specimen of air would
acquire if it were brought to standard pressure under ADIABATIC conditions.
If the absolute temperature and the pressure of the air be T and p respectively
and the standard pressure be P, the potential temperature 6 is given by
the relation
Potential temperature is related to the entropy <j> by the relation : —
$ = cp log, 6 + constant
where cp is the specific heat at constant pressure.
Precipitable Water. — Precipitable water is a term defining the quantity of
water vapour in a section of either the total atmospheric shell or an outer
section of that shell. It is the depth of water that would be obtained if all
the water vapour in a vertical column of 1 sq. cm. cross section were condensed
on to a horizontal plane of area 1 sq. cm., placed at the lower limit of the
section of atmosphere in question. Simpson (London, Mem. R. met. Soc., 3,
No. 21, 1928) has used the term in connexion with the amount of water vapour
in the stratosphere, basing his calculations on the expression : precipitable
water m (in millimetres) equals 1,000 pig where p is pressure in millibars
of saturated water vapour at the temperature of the base of the stratosphere,
Precipitation.—Used in meteorology to denote any aqueous deposit, in
liquid or solid form, derived from the atmosphere. Thus the figure repre­
senting the precipitation at a given station during a given period includes
not only the RAINFALL, but also DEW and the water equivalent of any solid
deposits (SNOW, HAIL or HOAR FROST) received in the RAIN-GAUGE. This
aggregate figure appears in British statistical publications under the single
heading RAINFALL.
PRESSURE
148
Pressure.— Pressure is the force per unit area exerted against a surface
by the liquid or gas in contact with it, and since the force on a unit area of
a surface at any particular point in a fluid is independent of the orientation
of the unit surface in the fluid, it follows that no specification of direction is
necessary in speaking of the pressure at any given point in the fluid. The
pressure of the atmosphere which is measured by the barometer, is produced
by the weight of the overlying air. The pressure exerted by the wind is very
small in comparison with that of the atmosphere ; a wind of force 6 on the
BEAUFORT SCALE exerts approximately one thousandth part of the pressure
of the atmosphere. See ATMOSPHERIC PRESSURE, EXTREMES, MILLIBAR.
Probability.— If in a large number N of trials, an event occurs « times
and fails to occur N — n times, the fraction n/N is called the probability
of occurrence of the event. If a coin is tossed, the ratio of heads to the
number of trials will have a general tendency to approach more and more
closely to \ as the number of trials is increased. Strictly speaking, probability
can only be discussed in dealing with a large number of trials. If a coin is
tossed 6 times, we cannot assume that the result will be 3 heads and 3 tails,
though if we were so placed that we had to adopt some definite figure, we
should assume 3 heads and 3 tails as the likeliest distribution. If the prob­
ability of occurrence of an event is 1/10, then we could not say that the
event should occur 10 times in 100 trials, nor even that it should occur
1,000 times in 10,000 trials, though the figure 1,000 could be accepted as
being relatively more reliable than the figure 10 in the first case.
In some cases we can resolve a complex event into a number of simpler
events, and it is important in that case to know how to evaluate the prob­
ability of the complex event from the separate probabilities of the simpler
events. Let the complex case be the simultaneous occurrence of two events
A and B, the probability of occurrence of A being a and the probability of
occurrence of B being b. Then the probability of the occurrence of both A
and B is ab, if, and only if, the events A and B are absolutely independent.
Unless the events are independent, this simple rule of multiplication of
probabilities is invalid. But, provided the events are all independent, the
rule can be extended to any number of events.
When we come to the problem of determining the probability of an event
solely from statistics relating to the frequency of its occurrence, since we
cannot make N indefinitely large, we have to be content with taking the
ratio of the observed frequency of occurrence to a finite number of trials N,
as the probability p.
If the probability of the occurrence of an event is p, the probability of
its non-occurrence is 1 — p. In a large number of trials, say N, the event
will occur Np times on the average, but for any series of N trials the actual
number of occurrences may differ from Np. It is useful to have some
guidance as to the extent of the variability to be expected in the result.
The PROBABLE ERROR of the result Np is -6745^/^v
i _
Or
(1 — p) with sufficient accuracy. An example may help to make
clear the utility of this relation. The ratio of the number of male children
born to the number of female children born is said to be 1 -05 : 1. Ifina
particular community 51,400 out of 100,000 children born were male, could
we deduce from this any abnormality in the community ? The expected
number of male children is 100,000 X
tf
is -6745
TC or 51,220. The probable error
100,000 x 1-05 x 1-0
, rt
„.
/ ————— .-0 ' n _. 2———— or 107. The deviation from the expected
result is 180, and is therefore, less than twice the probable error. If, on the
other hand, the number had been 51,800 males out of 100,000 births, the
deviation would have been about six times the probable error, and we should
be justified in regarding this as greater than would be produced by chance
and, therefore, as indicating some abnormality in the community concerned.
149
PROJECTION
A common way of expressing probability is to state it in the form of
" odds against " or " odds on " the occurrence of the event. If the probability
of an event is 1/10, the event fails nine tunes for each time it succeeds, and
this may be expressed by saying that the odds are 9 to 1 against the event.
See also NORMAL LAW.
Probable Error.—A quantity, usually denoted by r, such that the error
of a single observation is as likely to be within as without the limits i r.
In other words, there is an even chance that the error of a single observation
shall not exceed r. The probable error, therefore, gives a measure of the
closeness with which the observations are clustered about their mean value.
If xlt x2, . . xa are n observations whose mean value is x, the probable
error r is given by
/ s (* ~ *)'
r = 0-6745
V n-1
It is usually sufficiently accurate to replace n — 1 by n in the denominator.
If the observations are distributed in accordance with the NORMAL LAW OF
ERRORS the probability of the occurrence of an error as great as 2r is • 177,
of an error as great as 3r, -043, of an error as great as 4r, -007, of an error
as great as 5r, -001, and of an error as great as Gr, about -00005.
Prognostics.—See WEATHER MAXIM
Projection.—This term is used in connexion with maps in a sense wider
than that of geometrical perspective. It denotes any relationship estab­
lishing a correspondence between a domain of the earth's surface and a
domain of a plane surface, the map, such that to each point of one corre­
sponds one and only one point of the other. The projection is completely
represented by constructing, on the plane surface, a graticule formed by
two intersecting systems of lines, corresponding respectively to parallels of
latitude and meridians of longitude on the earth. The position on the map
of any features on the earth's surface is then determined by reference to
this graticule.
The scale of the map is the ratio of the distance between two neighbouring
points on the map to the corresponding distance on the earth. A perfect
map in which the scale is uniform throughout is not possible. A class of
projections termed " orthomorphic " or " conformal " have the property
that, at any point, the scale in all directions is the same, though varying
from point to point. This is equivalent to the property that the angle of
intersection of any two lines on the earth (such as an isobar and a meridian)
is preserved unchanged on the map, or the shape of any small area is preserved.
Orthomorphic projections are not much used generally, but owing to the
above properties, they enter into meteorological practice as base maps for
the representation of meteorological elements. They are recommended for
this purpose by a resolution of the International Meteorological Committee,
Salzburg, 1937. Three orthomorphic projections are recommended for the
purposes of synoptic meteorology :—
(a) the stereographic projection for the polar regions on a plane
cutting the sphere at 60° ;
(&) the Lambert orthomorphic conic projection for middle latitudes,
the cone cutting the sphere at 30° and 60° ;
(c) Mercator's projection for the equatorial regions with true scale
at 22|°.
The projection recommended in (a) is a special case of conical ortho­
morphic projection in which the meridians converge at their true angle and
in which the scale is correct on any one chosen parallel. It is particularly
suitable for the polar regions.
The projection recommended in (b) is very suitable, especially for middle
latitudes, and is used for the majority of the working charts of the Meteoro­
logical Office. The meridians are straight lines converging to the pole and
PSYCHROMETER
150
the parallels of latitude are circles centred at the pole. The scale is correct
at all points along two chosen parallels and any two meridians converge at
an angle which is a fraction of the angle between them on the earth, the
fraction depending solely on the choice of standard parallels. The spacing
of the other parallels is then uniquely determinate to secure the orthomorphic
property. The scale is somewhat too low between the standard parallels
and increases rather rapidly outside them.
The projection recommended in (c) is another special type of orthomorphic.
projection in which the angle between the meridians is zero. The meridians
are equally spaced parallel straight lines and the parallels of latitude are
straight lines at right angles to the meridians and spaced so as to secure
the orthomorphic property. This projection is most suitable for the equatorial
zone.
Conical and zenithal projections can be made with other than ortho­
morphic properties, by suitably altering the spacing of the parallels. Preser­
vation of areas may be secured or a compromise between the equal area and
orthomorphic properties.
The latter is the case in Clarke's zenithal projection used in the Daily
Weather Report (British Section) of the Meteorological Office.
There are, in addition, many other projections, each to serve a special
purpose. Of these Mollweide's " Equal-Area Projection " is often useful
when a world map is required. The whole globe is represented within an ellipse
whose major axis is twice the minor axis.
(For further information see " Map Projections," by A. R. Hinks, Camb.
Univ. Press. The mathematical properties of orthomorphic projections are
to be found under " conformal representation " in books on the theory of
functions of a complex variable.)
Psychrometer.—An alternative name for the dry- and wet-bulb hygrometer.
Two similar thermometers are employed, one, the " dry " bulb, reading the
air temperature, while the other, the " wet " bulb, whose bulb is covered
with muslin wetted with pure water, gives the " temperature of evaporation.'*
For a given value of the temperature, the wet bulb reads lower than the dry
bulb by an amount depending on the humidity and the rate of flow of
air over the bulbs. The connexion in the simplest form may be written
x = / - Ap(t-t')
where x = vapour pressure under the conditions of observation,
/ = saturation vapour pressure at the temperature (f) of the wet bulb,
p = the pressure of the air,
/ = the temperature of the dry bulb,
t' = the temperature of the wet bulb,
A = a " constant."
In the " Hygrometric Tables " issued by the Meteorological Office for use
with " light airs " the constant A is given the value 0-444, for wet-bulb
temperatures above 32° F. and 0'400 below that temperature. In the
edition published in 1927 (reprinted 1931) the saturation vapour pressure
over water is taken as the standard at all temperatures, even below 32° F.
In a supplement issued in January 1939 revised tables are given, based on
the principle that the saturation vapour pressure over ice is taken as the
standard in all cases where the saturation vapour pressure is less than that
corresponding with a temperature of 32° F. A number of other tables have
been published for the purpose of ascertaining the relative humidity, dewpoint, etc., from the readings of the two thermometers under various conditions.
In the Assmann psychrometer, a definite rate of ventilation is secured
by drawing the air over the bulbs by the agency of a fan driven by clockwork.
The thermometers are mounted in a plated metal frame designed to secure
immunity from errors due to solar radiation. In the sling or " whirling "
psychrometer (see SLING THERMOMETER), a similar degree of ventilation is.
151
RADIATION
secured by whirling the thermometers which are mounted in a frame
resembling a " policeman's rattle." For use on aircraft psychrometers have
been designed to utilise the motion of the aeroplane itself for ventilation
purposes.
Pumping.—Unsteadiness of the mercury in the barometer caused by
fluctuations of the air pressure produced by a gusty wind, or due to the
oscillation of a ship.
Purple Light.—Shortly after the sun has set below the western horizon
a brighter patch appears on the darkening sky about 25° directly above
the position where the sun has disappeared. This patch appears brighter
as the sky darkens and takes on a purple tone. The patch expands into
a disc and when the sun is about 4° below the horizon it reaches its maximum
brilliancy, when it may be so bright that white buildings in the east which
are lit up by it, glow with a purple colour which corresponds to the afterglow
seen on the peaks of snow-covered mountains. The disc of purple light
sinks downwards at twice the rate at which the sun sinks while at the same
time its radius expands and its light becomes less intense. It finally sets
behind the bright segment of the twilight arch. Occasionally when the
first purple light has passed below the horizon, the phenomenon repeats
itself with less intensity. The second patch of light appears at a slightly less
altitude than the first but otherwise follows the same course.
Another coloured band is to be seen in the east at sunset immediately above
the SHADOW OF THE EARTH. This band called the counter or anti-twilight
is golden when the shadow is on the horizon. As the shadow rises the band
acquires a reddish tint passing through reddish grey, purple violet, puiple
into red and orange yellow and, finally, green, rapidly increasing in breadth
and brightness as it does so. The upper boundary is quite indefinite. The
band reaches its maximum height and brightness about the time when the
sun is 2-3° below the horizon and the western purple light is just becoming
visible. After this the anti-twilight disappears and it usually ceases to be
recognisable when the sun is 4° below the horizon.
Sometimes there is a marked blue band at the base of the anti-twilight.
In contrast to the western purple light the brightness of the anti-twilight
increases with the height of the observer.
Pyranometer.—See PYRHELIOMETER.
Pyrgeometer.—See PYRHELIOMETER.
Pyrheliometer.—An instrument for measuring the rate at which radiant
heat is received from the sun. In Angstrom's form the rate of absorption
of heat by a thin strip of blackened platinum, when exposed normally to
the sun's rays, is ascertained by measuring the electric current necessary
to heat a similar strip to the same temperature. The rate of generation of
heat in the strip in the electrical circuit is then equal to the rate of absorption
of heat by the strip exposed to the sun. In Abbot's " silver disc " pyrheliometer, the rise of temperature in a silvered disc exposed to the sun's rays
is measured directly and the intensity of the radiation is determined therefrom
by reference to the calibration data of the instrument. The PYRANOMETER,
devised by Abbot and Aldrich, measures outgoing radiation (nocturnal
radiation) as well as sky radiation. The nocturnal radiation can also be
measured by Angstrom's PYRGEOMETER, in which use is made of the fact that
the radiation from a gilded strip of manganin is less than that from a blackened
strip.
Radiation.— Units.—The intensitv of radiation is measured by the energy
which crosses a certain area in a given time. In physics the unit adopted is
generally one erg per cm. 2 per sec. For meteorological purposes a larger unit
is required and one milliwatt per cm. 2 is found appropriate. The relation
between these units is simple. Moreover, the latter unit can be brought
into relation with the engineers' unit of power, the kilowatt, by adopting the
RADIATION
152
square decametre as unit of area. In most meteorological work the energy
is expressed in terms of the gram-calorie (see CALORIE), the energy required
to raise the temperature of a gram of water by one degree centigrade.
Theoretically this is not a good unit as it can only be made definite by
specifying the actual range of temperature. The capacity for heat of water
varies from 4-219 joules per gram per degree at 0° C. to 4-173 at 35° C.
In meteorology the 15° calorie and the 20° calorie have both been used.
The equivalents are 4-184 joules and 4-180 joules respectively. Fortunately
the difference is of little practical importance. The relations between the
units of radiation are : —
i
-
Kw-
cm. 2 ~ dm. 2 ~~
cm. 2 sec.
___
cm. 2 mm.
cm. 2
. cal.
1
gm. cal.
mw.
_
_
cm.2 day
1440 cm. 2 min.
cm. 2
These units are suitable for the measurement of the total radiation
received by a surface from the sun or from the whole sky. For the measure­
ment of the radiation from a part of the sky W. H. Dines adopted the
convenient practice of giving the radiation which would be received on a
flat surface from the whole sky if the intensity were uniform. When the
intensity is uniform, the radiation per unit of solid angle is equal to
(2/71) X (the radiation received on a flat surface).
Black-body radiation. —The colours attributed to solid objects depend on
their appearance when daylight falls on them. A white surface scatters a
large proportion of the daylight : a dull black surface absorbs the daylight.
Little light is absorbed by a bright metallic surface.
According to the theory of exchanges all solids give out radiant heat.
When a solid is hotter than its surroundings it gives out more heat than it
absorbs. The radiation from a burnished metal is much less than that from
a dull surface at the same temperature, and experiment indicates that at
moderate temperatures the strongest radiation is given out by a black
surface.
When solids are heated so that they become red hot or even white hot
there are still differences between the amounts of radiation which they emit.
In general the solid which is black at ordinary temperatures continues to
give out most radiation at higher temperatures. Experiment suggests that
there is an upper limit to the radiation which can be emitted by a surface
of specified area at any given temperature. The ideal substance which would
at every temperature emit the greatest possible amount of radiant energy
is known as a " perfect radiator " or " perfectly black " substance.
It has been demonstrated that the radiation which passes out through
the narrow mouth of a cavity in a solid at uniform temperature is equivalent
to that from a perfect radiator at that temperature. The radiant energy
emitted by an element dS of the surface of a perfect radiator is given by a
simple formula, found empirically by Stefan and afterwards demonstrated
theoretically by Boltzmann. The formula is
total radiation = a T* dS
T being the absolute temperature and a a constant. If T is measured in
centigrade degrees from — 273- 1° C. the value of a is
5-709 x 10~8 ergs per cm. 2 per sec.
This is equivalent to 5-709 x 10~12 watts per cm. 2 and to 82 x 10~12
gram-calories per cm.2 per min. The same formula serves to give the
flux of radiation passing directly or obliquely across an area dS parallel to
the radiating surface and a small distance from it.
RADIATION
153
DISTRIBUTION OF ENERGY IN
BLACK BODY SPECTRUM
D
Scale A for £00°A (Stratosphere) Totel • 9M3
B
C
D
-
(Ground)
300°A.
I,00(TA. (Red Hob)
(Sun)
G,00(TA.
•• =46-Z -• —
- -5,709--- -TjWqpOO—-
FIG. 24.
The distribution of energy in the spectrum.—The familiar fact that a piece
of metal changes colour as it is heated in a furnace is an indication that the
distribution of radiant energy in the spectrum is changing. Wien's displace­
ment law tells us that the graph showing the relation of energy to wave­
length for a perfect radiator at one temperature will serve to give the relation
at any other temperature. The wave-length scale has to be changed in the
inverse ratio of the temperatures, whilst the energy scale is changed by the
fifth power of this inverse ratio.
Planck's radiation formula.—The laws of Stefan and Wien can be deduced
from the classical laws of thermodynamics. The actual distribution of energy
in the black-body spectrum cannot be deduced from these laws. The
specification of this distribution was given by Planck. In doing so he made
one of the most important and difficult steps in the advance of science, by
propounding the " Quantum Theory."
Planck's distribution law is illustrated in Fig. 24 for a black body at
6,000°A. The distribution of energy is comparable with that in solar radiation.
The visible spectrum occupies the range from 0-4(ji, (violet) to 0-77(i (red)
or in Angstrom units from 4.000A to 7,OOOA.
Other scales in the diagram illustrate the distribution of infra-red radiation
from cold surfaces and from a red-hot surface. Note that nearly all the energy
emitted from the red-hot surface is in the infra-red. The curve ordinate
corresponding with 0-75(i on scale C should be only 0-06 per cent, of the
maximum ordinate which corresponds with SIL.
Radiation and the atmosphere.—The principal gases of the atmosphere,
oxygen and nitrogen, are transparent to nearly all the radiation of the solar
spectrum and also to the long-wave radiation such as is given out by a body
RADIATION
154
at the temperature of the ground. Carbon dioxide absorbs long-wave radiation
with wave-length in the neighbourhood of 15(i. The most important absorbent
in the atmosphere is, however, water vapour. The water vapour absorbs
some of the solar radiation in the visible spectrum (so giving rise to " rainbands " in the spectroscope) and much of the infra-red radiation. Most
long-wave radiation is absorbed by water vapour but there is a short range
of wave-lengths between 8(i and 12^ for which the vapour is transparent.
Since gases emit and absorb radiation of the same wave-lengths, the
radiation which is returned by the atmosphere to the ground comes mostly
from the water vapour and it is found by observation that the strength of
the radiation from the sky after dark depends on the amount of water vapour
present as well as on its temperature.
Thus the absorbing power of water vapour plays an important part in
the meteorology of the lower atmosphere. It is believed to be equally
important in the meteorology of the upper atmosphere as it is mostly through
the absorption of long-wave radiation that the temperature of the stratosphere
is maintained.
The part played by water in the form of clouds in regulating the tem­
perature of the globe has not been fully realized until recently. Clouds have
a high reflecting power and it is estimated that the light reflected from the
clouds together with light reflected from the sea and from the air accounts
for half of the radiation received by the planet from the sun. This half passes
out into space as short-wave radiation. The other half is absorbed by the
planet; the equivalent energy passes out again eventually as long-wave
radiation. The value of this fraction depends on the average amount of
cloud over the globe. Simpson* has pointed out that an increase in radiation
from the sun would lead to an increase in the amount of cloud and, therefore,
to an increase in the proportion of light and heat reflected. There is almost
an automatic control of the temperature of the globe. It is instructive to
notice that Venus, which is nearer to the sun than is the earth, is continually
mantled in clouds whilst Mars, which is further away, is devoid of cloud.
Only the earth enjoys such conditions that the cloud-thermostat can be
effective.
Terrestrial radiation.—The cooling of the ground at night is mainly due
to the excess of the outward radiation above the radiation which comes
down from the sky. The radiation from the ground is practically the same
as would be given out by a perfect radiator at the same temperature. (See
BLACK-BODY RADIATION.) It is convenient to regard it as divided into three
parts : (a) radiation in wave-lengths between 5'5(z and ?JJL and all radiation
of longer wave-length than 14(i. To radiation of this sort water vapour is
opaque; (6) radiation in wave-lengths between 8£[j, and ll(i: to such
radiation water vapour is transparent; (c) radiation in intermediate wave­
lengths. To this water vapour is semi-transparent.
The radiation of the (a) type is caught quite close to the ground. There
is always enough water vapour to send back a full measure of this radiation
so that the inward and outward flows of radiant energy balance. The radiation
of the (b) type passes (if the air is clear and there are no clouds) right through
the atmosphere to outer space. There is no compensating inward radiation.
The proportion of (c) radiation absorbed by the atmosphere depends on the
quantity of water vapour in the atmosphere and the strength of the return
radiation is affected in the same way.
According to the observations made by A. Angstrom in Algeria and
California the ratio of the incoming radiation from the sky to that emitted
by a black body at air temperature varies between 62 per cent, for a vapour
pressure of 4 mb. and 74 per cent, for a vapour pressure of 16 mb.
* " Further studies in terrestrial radiation." London. Mem. R. met. Soc., 3, No. 21, 1928.
155
RADIO-SONDAGES
If the ground were at the same temperature as the air and the vapour
pressure 10 mb. the resultant outward flow from the ground would be
32 per cent, of black-body radiation. Actually the ground is cooled below
air temperature and there is a corresponding reduction in the radiation given
out. Thus with the ground 5° C. below air temperature the radiation emitted
from the ground is reduced to about 93 per cent, of that of a black body at
air temperature and the resultant outflow of radiation is reduced from 32 per
•cent, to 25 per cent.
The conditions favourable for cooling of the ground at night are :—
(1) A cloudless sky. When the sky is covered with clouds at moderate
heights the radiation from the clouds in types (b) and (c) is almost equiva­
lent to the radiation of the same types from the ground.
(2) Dry air—so that the return radiation of type (c) may be slight.
(3) Absence of wind. The effect of wind is to bring more air into
contact with the ground. The air gives heat to the ground and so prevents
it from cooling so much. On the other hand the cooling of the lowest
layers of the air produces stratification and reduces turbulence so that
the wind can continue at considerable heights and glide over the quiet
air near the ground. Thus it is equally true that the air is calm because
the ground is cold and that the ground is cold because the air is calm.
(4) Low conductivity of the ground. The heat conducted from below
tends to keep up the temperature of the surface. In this connexion it
may be noticed that snow being a bad conductor the surface of snow can
attain very low temperatures.
The most conspicuous signs of effective terrestrial radiation are the
deposit of DEW and HOAR-FROST.
Terrestrial-radiation thermometer.—The " grass minimum thermometer "
is sometimes referred to as a " terrestrial-radiation thermometer." Low
readings of such a thermometer are a sign that terrestrial radiation has been
•effective but there is no rule for deriving from the thermometer readings a
numerical estimate of the strength of the radiation. The thermometer is
placed above the grass so that its temperature is not affected by conduction
from the ground. It is to be expected that the thermometer will be a little
cooler than the grass blades near by.
Ultra-violet radiation.—When a spectrum is photographed it is found
that the limits of the photograph are by no means the same as the visible
limits of the spectrum. Probably the red end of the spectrum is cut off in
the photograph. On the other hand the photograph extends considerably
beyond the violet. The photographic paper is said to have been affected by
ultra-violet radiation. The prolongation of the solar spectrum beyond the
violet was first noticed by Sir John Herschel in throwing the spectrum on
turmeric paper. The prolongation was yellow. Herschel's experiment was
an example of how certain substances become " fluorescent " when illuminated
by ultra-violet light.
The wave-lengths of ultra-violet light are shorter than those of visible
light, the limit of which is about 0-4(jt. On the other hand there is no ultra­
violet of very short wave-length in the light which reaches us from the sun.
The solar spectrum is cut off at about 0-3(1. It is believed that the ultra­
violet of shorter wave-length is absorbed by ozone some 30 kilometres or
more above the ground.
One of the effects of ultra-violet light is the ionization of gases. It is
probable that the high electrical conductivity of the " Heaviside layer " in
the upper atmosphere is produced in part by ultra-violet light from the sun.
Radio-meteorograph.—See RADIO-SONDES.
Radio-sondages (Radio-soundings).—The first successful attempt to transmit
radio signals from apparatus carried by a small balloon was made by P. Idrac
and R. Bureau in March, 1927. Since that date many forms of apparatus
RADIO-SONDES
156
designed to regulate such signals and report the temperature, pressure and
humidity of the upper atmosphere, have been invented. Most designers have
worked on Olland's principle in which the interval of time between two pulses
indicates pressure, the one between the next two, temperature and so on.
The next fundamental development was the use of the thermometric element
to vary the frequency of the radio signal by controlling the capacity of a
condenser as was done by Duckert in Germany and by Vaissala in Finland in
1932.
There are however objections to varying the radio-frequency of a trans­
mitter, and in designing the first English radio-sonde in 1937, Dr. H. A. Thomas
incorporated a continuous wave transmission modulated by two audio­
frequency notes of frequencies varying with the pressure and temperature
respectively. Variations of the pressure and temperature elements controlled
the self-inductance of the modulating circuit. These notes were separated
and their frequencies measured at the receiving station and converted from
previous calibrations to pressure and temperature readings. Later the
Kew radio-sonde was developed from the Thomas sonde, and has been in
general British use for several years. The Kew radio-sonde measures pressure,
temperature and humidity, and transmits the readings by means of the
modulated continuous wave technique. The circuits controlled by each
element are switched in alternately so that at any moment only one circuit
is modulating the transmission. As in the case of the Thomas sonde, the
frequencies are measured at the observing station and converted from previous
calibrations.
Certain wave-bands are allocated by international agreement for radiosoundings.
Radio-sondes.—These are specially designed instruments carried by free
balloons which transmit by radio the instantaneous values of pressure,
temperature and, usually, humidity in the atmosphere.
The outstanding importance of this method is that it enables the values of
pressure, temperature and humidity at different heights in the atmosphere to
be obtained in all conditions of weather, and at altitudes far greater than can
be reached by other means. The normal height to which it is possible to
obtain information by radio-sonde is about 60,000 ft. In favourable
conditions values can be obtained up to 80,000 ft.
Rain.—" Condensed moisture of atmosphere falling visibly in separate
drops" (Concise Oxford Dictionary). The distinction between rain and
DRIZZLE, though readily recognizable, is not easy to express in a precise form.
It has been suggested that the term rain should be limited to liquid
precipitation in which the diameter of the drops exceeds 0-5 mm. but this
figure has not been internationally agreed and it is almosi certainly too high.
Rain-band.—A dark band in the solar spectrum on the red side of the
sodium D lines, due to absorption by water vapour in the earth's atmosphere.
The band may be best seen when the spectroscope is pointed at the sky
rather than directly at the sun. The band is strengthened with increase of
water vapour and also when the altitude of the sun is low and its light has toshine through a greater thickness of air. Taken alone the rain-band is of
doubtful value as a prognostic of rain. Accurate measurements of the
absorption may be utilized however to determine the quantity of moisture in
the atmosphere whereas ordinary meteorological observations merely give
the quantity of vapour near the ground.
Rainbow.—A rainbow is seen when the sun shines upon raindrops. The
drops may be at any distance from the observer from a few yards to several
miles. When sunlight falls upon a drop of water some of the light enters
the drop, is reflected from the far side and emerges from the near side. The
light which is reflected in this way does not come out in all directions but only
in directions lying within about 42° from the direction of the sun. The
reflected light is most intense near the limit. Accordingly an observer lookingtowards the raindrops receives a certain amount of light from all directions
within 42° from the shadow of his head but most light along rays which make
about 42° with the central line. The limiting angle depends on the colour
RAINDROPS
157
•of the light and in so much as white light is compounded of light of different
spectral colours the observer sees a number of concentric arches of different
colours.
Some of the light falling on a drop does not emerge until after it has
been reflected twice. None of the twice reflected light which reaches an
observer makes an angle of less than 50° with the line to the centre of his
shadow. The colours of the outer bow formed in this way are in the reverse
order to those of the inner bow. The space between the inner and outer
bows appears darker than the space inside the inner or beyond the outer bow.
It is a common mistake to assume that the colours of all rainbows are
the same. The colouration depends on the size of the drops. Drops larger
than 1 mm. in diameter yield brilliant bows about 2° in width, the limiting
colour is distinctly red. With drops about 0-3 mm. in diameter the limiting
colour is orange and inside the violet there are bands in which pink pre­
dominates. With smaller drops supernumerary bows appear to be separate
from the primary bow. With still smaller drops about -05 mm. in diameter
the rainbow degenerates into a white fog bow with faint traces of colour at
the edges.
The wave theory of light has provided in the hands of Airy and Pernter
a satisfactory explanation of all the variations in the colour of rainbows.
Rainbows are not infrequently observed by moonlight but as the human
eye cannot distinguish colour with faint lights the lunar rainbow appears
to be white.
Rain Day.—A period of 24 hours, commencing normally at 9h. G.M.T., on
•which 0-01 in. or 0-2 mm. or more of rain is recorded. The following papers
deal with average numbers of rain days in the British Isles :—
(1) The distribution over the British Isles in time and space of the
annual number of days with rain. British Rainfall, 1926, p. 260-79.
(2) The distribution over the British Isles of the average number of days
•with rain during each month of the year. London, Quart. J. R. met. Soc.,
54, 1928, p. 89-101. See also WET DAY.
Raindrops.—Size and rate of fall of raindrops.—The size of raindrops
can be measured, if, for example, a shallow tray containing dry plaster of
Paris is exposed for a few seconds during rain. Each drop which falls into
the tray will make a plaster cast of itself which can easily be measured.
A better method is to collect the drops upon thick blotting paper. If, while
still wet, the paper is dusted over with a dye powder a permanent record
will be obtained consisting of circular spots whose diameter is a measure
of the size of the drops. By comparing the diameters of the discs produced
by raindrops with those produced by drops of water of known size, the
amount of water contained in the former can be found. The following
table contains some results obtained by P. Lenard* in this way at nine
different times :—
Drops
Diameter
Number of drops per square metre per second
Volume
mm.1
0-066
0-523
1-77
4-19
8-19
14-2
22-5
33-5
47-8
65-5
(1)
1,000
200
140
140
0
0
0
0
0
0
(2)
1,600
120
60
200
0
0
0
0
0
0
(3)
129
100
73
100
29
57
0
0
0
0
(4)
60
280
160
20
20
0
0
0
0
0
(5)n
0
50
50
150
0
200
0
50
0
0
(6)
100
1,300
500
200
0
0
0
0
200
0
(7)
514
423
359
138
156
138
0
0
101
0
(8)
679
524
347
295
205
81
28
20
0
0
Total number
1,480
1,980
486
540
500
2,300
1,840
2,190
500
Rate of rainfall
(mm./min.)
• " Uber Regen."
0-09
0-06
0-11
0-05
0-32
0-72
0-57
0-34
0-26
mm.
0-5
1-0
1-5
2-0
2-5
3-0
3-5
4-0
4-5
5-0
in.
•019
•039
•059
•079
•098
•118
•138
•157
•177
•196
Met. Z., Braunschweig, 21, 1904, pp. 249-62.
/O\
(%
233
113
46
7
0
32
39
0
25
RAINDROPS
158
(1) and (2) refer to a rain " looking very ordinary " which was general
over the north of Switzerland. The wind had freshened between (1) and (2).
(3) Rain with breaks during which the sun shone.
(4) Beginning of a short fall like a thunder shower. Distant thunder.
(5) Sudden rain from a small cloud. Calm ; sultry before.
(6) Violent rain like a cloud-burst, with some hail.
(7), (8) and (9) are for the heaviest period, a less heavy period and the
period of stopping of a continuous fall which at times took the form of a
cloudburst.
Probably the best method of measuring the sizes of small raindrops is
that of Nolan and Enright (Sci. Proc. R. Dublin Soc., 17, 1922, p. 3). Glass
microscope slides are prepared by spreading on each of them a layer of thick
dark oil of density 0-9. When one of these slides is exposed to rain the drops
falling on it pass into the oil layer and are there suspended in the form of
perfect spheres, sinking very slowly. The rate of movement through the
oil is so slow that the smallest drops do not reach the bottom of the layer
until after 48 hours. The diameters of the drops are easily and accurately
measured under a low-power microscope.
It is apparent from the table that by far the greater number of drops
have a diameter of 2 mm. or less. In short showers, especially those occurring
during thunderstorms, the frequency of large drops is much greater. In
such showers the diameter of the largest drops appears to be about 5 mm.
We shall see later that it is impossible for a drop, whose diameter exceeds.
5 • 5 mm. or rather less than a quarter of an inch, to fall intact.
The rate at which a raindrop, or any other object, can fall through still
air depends upon its size. When let fall its speed will increase until the air
resistance is exactly equal to the weight, when it will continue to move at
steady speed. (See EQUILIBRIUM). The manner in which this " terminal
velocity," as it is called, varies with
the size of the raindrops is shown in
Fig. 25 on which some of the actual
observations made by Lenard have
been plotted.
7
We may consider this diagram
in another way.
The frictional
resistance offered by the air to the
passage of a drop depends upon the
relative motion of the two, and it is
of no consequence whether the drop
is moving and the air still, or the air
moving and the drop still, or both
air and drop moving if they have
different velocities. The velocities
given in the tables are those with
which the air in a vertical current
must rise in order just to keep the
drops floating, without rising or
falling. The above results were, in
fact, actually determined by Lenard
in this way, by means of experi­
ments with vertical air currents on
drops of known size. We see that
beyond a certain point the terminal
velocity does not increase with the
size of the drops but tends to de­
crease. This is due to the fact that
the drops become deformed, spread­
ing out horizontally, with the result
ZO 30 40 50
that the air resistance is increased.
DIAMETER OF DROP (mm)
For drops greater than 5-5 mm.
FIG. 25
diameter,
the deformation is
sufficient to make the drops break up before the terminal velocity is reached
RAINFALL
159
An important consequence of Lenard's results is that no rain can fall
through an ascending current of air whose vertical velocity is greater than
8 m./sec. In such a current the drops will be carried upwards, either intact or
after breaking up into droplets. There is good reason for believing that
vertical currents exceeding this velocity frequently occur in nature.
On account of their inability to fall in an air current which is rising faster
than their limiting velocity, raindrops formed in these currents will have
ample opportunity to increase in size. These large drops can reach earth in
two ways : either by being carried along in the outflow of air above the
region of most active convection, or by the sudden cessation of or a lull in
the vertical current. The violence of the precipitation under the latter
conditions may be particularly disastrous. See also CLOUDBURST AND HAIL.
Rainfall.-—The total product of PRECIPITATION or CONDENSATION from
the atmosphere as received and measured in a RAIN-GAUGE. Ultimately all
rainfall is due to condensation of AQUEOUS VAPOUR. Condensation occurring
at a considerable height above ground gives rise initially to CLOUDS, and
finally, under conditions which are still not completely understood, to aggre­
gates of water droplets or ice particles of sufficient size to fall with appreciable
speed and to survive the loss by EVAPORATION during the journey to the
ground. RAIN, SNOW and HAIL precipitated in this manner make up by far
the greater part of the total rainfall, but there are also small additions due
to the deposition of DEW, HOAR-FROST or RIME on to the collecting surface of
the rain-gauge.
Three types of rainfall are recognized, depending upon the process by
which the dynamical cooling leading to cloud formation is brought about.
The types are OROGRAPHIC, CYCLONIC and CONVECTIONAL and these are
discussed under their separate headings. While orographic and cyclonic
rains are often very persistent and widespread, convectional rains are typically
intense and of short duration. Both orographic and cyclonic rains are rather
more frequent in winter, while convectional rains are more common in summer.
The average annual rainfall at any station is the mean annual fall over a
long period, often the 35 years, 1881-1915. Annual averages are included for
many stations in the annual volumes of British Rainfall. The monthly and
annual averages at some 570 stations are set out in Section V of the " Book
of Normals of Meteorological Elements for the British Isles." Over the
British Isles the average annual rainfall varies from 20 in. along the Thames
estuary to over 150 in. in the English Lake District, Snowdonia and parts of
the Western Highlands of Scotland. Maps showing the average distribution
in the year and the individual months have been included in the " Rainfall
Atlas of the British Isles," published by the Royal Meteorological Society in
1926. Details of the largest averages are given in a note under the title of the
" Wettest places in the British Isles " in the Meteorological Magazine, 72,
1937, pp. 142-4. The average monthly rainfall 1881-1915 for each month
of the year of the larger divisions and of the British Isles is set out below.
England
Wales
England
and
Wales
Scotland
Ireland
British
Isles
January
February
March
in.
2-69
2-34
2-47
m.
4-72
3-94
3-82
in.
2-99
2-57
2-67
in.
4-90
4-18
4-05
in.
4-07
3-53
3-36
in.
3-78
3-26
3-22
April
May
June
1-98
2-19
2-33
2-96
2-95
3-05
2-12
2-30
2-44
2-99
3-01
2-83
2-75
2-75
2-82
2-52
2-61
2-64
July
August
September
2-75
3-11
2-37
3-60
4-71
3-51
2-87
3-35
2-54
3-78
4-51
4-00
3-37
4-20
3-13
3-25
3-88
3-09
October
November
December
3-69
3-19
3-56
5-63
5-25
6-00
3-97
3-49
3-92
4-90
5-29
5-88
4-08
4-28
4-96
4-25
4-19
4-72
32-67
50-14
35-23
50-32
43-30
41-41
Year
160
RAINFALL
It will be noticed that over the British Isles as a whole December is the
wettest month and April the driest. The three months October, November
and December receive nearly twice as much rain on the average as the three
months April, May and June. There is also a subsidiary maximum in August,
making that month the fourth wettest of the year. The seasonal distribution
in the various divisions of the British Isles is very similar. In July and
August the rainfall is actually greater in the east of England than many earlier
months of the year, and in some places those months are the rainiest in the
year on account of the frequency of summer thunderstorms. Over the east
and south of England generally the rainiest month is October, while for the
west and north of Great Britain it is December or January.
Extremes of rainfall can be conveniently considered for the various
units of time, e.g. the year, month, day and even for shorter periods. In
the case of the month and year, the distribution of the extreme values is largely
controlled by the configuration of the land, while for shorter periods the
largest values on record occur in the drier regions of central and eastern
England, where convectional rains are more common, (a) The wettest year
over the British Isles as a whole was 1872, and the driest year 1887. The
wettest and driest years on record have not been the same, however, in all
parts of the British Isles, and maps showing the wettest and driest years in
all parts of the country have been published (see London, Quart. J. R. met.
Soc., 52, 1926, pp. 237-48). Annual totals exceeding 240 in. were recorded
at The Stye, at the head of Borrowdale in Cumberland, in 1872 and 1923,
at Ben Nevis Observatory, in Inverness, in 1898, and at Llyn Llydaw on
Snowdon in 1909. The rainfall in 1921 in the south-east of England was by
far the smallest on record, as little as 10 in. being recorded at Margate, (b)
The wettest month over the British Isles as a whole was December, 1876, and
the driest months February, 1891 ; June, 1925 ; March, 1929; February,
1932 and April, 1938. The largest actual values at individual stations are
set out below :—
Station
Snowdon (Llyn Llydaw)
Borrowdale (The Stye)
Ben Nevis Observatory
County
Carnarvon
Cumberland . .
Inverness
Month
October, 1909
January, 1872
December, 1900
Amount
..
in.
56-54
50-05
48-34
Several stations recorded no rain in February, 1891 ; April, 1893;
September, 1894; February, 1895; July, 1911; April, 1912; June, 1921;
June, 1925 ; March, 1929 ; March, 1931 ; February, 1932 ; February, 1934
and April, 1938. The stations with no rain were mainly in south-eastern
England. In February, 1891, some 270 stations situated in central and
south-east England measured no rain for the month, while in June, 1925,
no rain was recorded over an area of 6,410 square miles, or an area equal
to about 85 per cent, of the total area of Wales.
The distribution of the extreme annual values is very largely controlled
by the configuration of the land, the largest values occurring in the mountainous
regions and the smallest on the plains. The distribution of the monthly
extremes reveals a second factor, viz., that the south-east of England is more
liable to periods of little or no rain, while in the north-west rain falls more
frequently. Thus, at Eallabus, in Islay, on the west coast of Scotland, rain
fell every day from August 12 to November 8, 1923, a period of 89 days,
or nearly three months, the average annual rainfall at Eallabus being only
about 50 in. a year. This second factor is shown very clearly in monthly and
annual maps of the number of days with rain. They indicate that in general
there is a steady increase in the number of days with rain from the south-east
to north-west, even at stations with the same annual fall. The least number
of days recorded in any year was in 1921, when there were less than 100 days
of rain over a well marked area in the neighbourhood of the Thames Estuary.
In 1923 the north-west of Ireland had more than 300 days with rain and
Ballynahinch Castle, in Connemara, recorded 309 rain days in the same year.
161
RAINFALL
A map of the average annual number of rain days was published in British
Rainfall, 1926, p. 264, and maps of the average number in each month in
London, Quart. J. R. met. Soc., 54, 1928, pp. 90-101.
The longest periods on record in these Islands with no rain occurred in
1893, during the famous spring drought, when some 20 stations in the south­
east of England, mostly in Kent and Sussex, recorded no rain for a period of
50 days or more. Locally in this district there was a two months' drought
from March 17 to May 16. The year 1893 was unprecedented for periods of
little or no rain.
The largest falls on record for one day (9h. to 9h.) are set out below :—
County
Somerset . .
,,
>i
i»
Glamorgan
Inverness . .
Cumberland
Station
Bruton (Sexey's School) . .
Cannington (Brymore House)
Bruton (King's School) . .
Aisholt (Timberscombe) . .
Rhondda (Pont Lluest Wen)
Loch Quoich (Kinlochquoich)
Borrowdale (Seathwaite) . .
Amount
Date
in.
9-56
9-40
8-48
8-39
8-31
8-20
8-03
June 28, 1917.
August 18, 1924.
June 28, 1917.
June 28, 1917.
November 11. 1929.
October 11, 1916.
November 12, 1897.
The average annual rainfall for the first four stations is 30 to 35 in.
while that for the sixth and seventh stations exceeds 100 in.; so that with the
short interval of the day the influence of configuration over the distribution
of extreme values has practically disappeared.
The most widespread heavy rain on record occurred in East Anglia on
August 25 and 26, 1912, when 1,939 square miles received more than 4 in.,
corresponding with a volume of rainfall of 154,133 million gallons.
Details of heavy falls on rainfall days, 1865 to 1934, are given in British
Rainfall, 1934, pp. 266^83.
Of the list of heavy falls in short periods, it is only possible to quote a
few examples. The fall of 1-25 in. in five minutes reported at Preston in
Lancashire, on August 10, 1893, gives the largest rate on record, viz., 15 in.
per hour. The largest fall in half an hour is 2-90 in. recorded at Cowbridge
in Glamorganshire on July 22, 1880, and in one hour that of 3-63 in. at
Maidenhead on July 2, 1913. The heavy rain of 4-65 in. in two and a half
hours on June 16, 1917, at Campden Hill in Kensington, is worthy of mention,
since this amount is the largest on record for London for a rainfall day.
It has been estimated that during the unprecedented storm at Cannington,
near Bridgwater, on August 18, 1924, as much as 8 in. of rain (and hail)
fell in 5 hours.
Intense falls are classified in British Rainfall as " noteworthy," " remark­
able " and " very rare." A revised classification and other details are given
in British Rainfall, 1935, pp. 262-80.
Variations of annual rainfall.—In the " Rainfall Atlas," to which reference
has already been made, maps are given showing the fall of each of the
56 years in the series 1868 to 1923, as a percentage of the average. Similar
maps are published in the recent volumes of British Rainfall and the whole
series gives a history of the rainfall fluctuations over the country during
the last 70 years.
While the variations of rainfall from point to point in any one year or at
the same point in successive years can be shown clearly on maps the variations
from year to year of the total annual rainfall for any large area as a whole
are shown more effectively by figures. The following table is taken from the
Meteorological Magazine for June, 1923, and brought up to date by reference
to the recent volumes of British Rainfall.
(90565)
F
RAIN-GAUGE
162
Annual rainfall for each year, 1868 to 1937, expressed as a percentage of the
average of the 35 years, 1881 to 1915
Englandand
Year
Wales
Scotland
Englandand
BrIslesitish
Year
Ireland
Wales
Scotland
Ireland
!A
.83
-4-»
•c
«
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
99
105
82
97
144
89
95
114
117
126
110
109
113
109
127
107
88
101
117
74
98
94
90
111
94
83
108
96
93
101
87
94
109
88
84
113
100
80
92
134
105
108
99
114
131
92
91
86
95
113
106
104
91
94
80
93
85
103
100
96
97
102
95
99
95
109
106
119
94
86
104
98
95
98
128
94
98
99
104
123
96
96
95
100
113
110
98
92
109
77
98
93
97
98
98
82
104
96
94
114
98
102
115
96
92
104
102
84
96
137
94
99
107
113
127
102
100
102
103
120
108
94
96
110
77
97
92
95
105
95
86
105
95
95
103
95
99
113
92
87
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
128
89
86
101
99
91
105
113
94
125
98
108
110
114
98
107
105
109
70
105
113
120
106
102
123
115
100
117
109
103
81
95
113
109
110
129
93
97
111
104
99
101
105
99
108
93
103
96
117
97
106
93
105
99
94
120
105
100
111
114
123
101
109
104
108
80
110
108
94
91
124
101
89
93
99
96
93
109
96
108
106
108
102
114
99
111
91
111
88
92
110
122
99
100
108
126
106
116
111
95
77
105
98
105
103
127
93
89
101
100
94
101
110
96
116
99
107
105
115
97
10&
98
10£
82
100
114
117
104
10£
11&
118
101
115
109
102
80
100
109
105
104
In this table the average rainfall for the 35 years 1881 to 1915 for each
district dealt with is taken as 100, and the general rainfall for each year
of the series is stated as a percentage of that amount. The table shows that
for the British Isles as a whole, wet and dry years have alternated irregularly.
There was a run of nine consecutive wet years from 1875 to 1883. Then,
commencing in 1891, there was a remarkable series of one wet year followed
by two dry, repeated five times in regular succession. During the 19 years,
1884 to 1902, only five had a rainfall above the average. Subsequently a
definite wet period set in and during the 29 years, 1909 to 1937, only six years
can be regarded as dry. It is interesting to note that one of the driest years
on record, namely 1887, occurred in the run of generally dry years, while the
other dry years, 1921 and 1933, occurred in a definite wet period.
Bain-gauge.—An instrument for measuring rainfall. In the " Snowdon,"
" Meteorological Office " and allied patterns, a funnel, usually 5 in. or 8 in.
in diameter, is used to collect the rain. A brass tube of fairly narrow bore,
to minimise evaporation, conducts the collected water into the collecting
vessel which may be a bottle or a copper can. The funnel is supported by
means of an outer can preferably of sheet copper. The rim is of stout brass
with a sharp bevelled edge and the funnel is deep, with vertical sides to
minimise errors due to splashing and to retain snow. The gauge is mounted
in an open situation with its rim 12 in. above ground level.
163
REDUCTION TO SEA LEVEL
In self-recording gauges, such as the HYETOGRAPH, the collected rainfall
is usually made to raise a float to which is attached a pen writing on a chart
wrapped round a clock-driven drum. Various methods, involving much
ingenuity, have been adopted to secure sensitiveness and to return the pen
to the zero line after a definite amount of rain has fallen.
Rain Shadow.—An area with a relatively small average rainfall due to
sheltering by a range of hills from the prevailing rain-bearing winds. The
phenomenon is noticeable in rainfall maps for months in which unusually
strong W. winds have predominated, e.g. to the east of Wales.
Rain Spell.—A period of at least 15 consecutive days to each of which is
credited -01 in. of rain or more ; thus the definition of the term " rain spell "
is analogous to that of the term " absolute drought." (See DROUGHT.)
During the 62 years, 1858-1919, there were seven rain spells at Camden
Square (London). See also WET SPELL.
Reaumur.—Rene Antoine Ferchault de, died 1757, whose name is given
to a scale of temperature now almost obsolete. On it the freezing point of
water is zero, and the boiling point 80°.
Recurvature of Storm.—This expression refers to the recurvature of
the track of a tropical CYCLONE, which is a typical feature of the great
majority of these phenomena. It is also known as the " recurve." In the
northern hemisphere a tropical cyclone after proceeding in a more or less
westerly direction recurves and normally takes a north-easterly direction ;
in the southern hemisphere the final direction is normally south-easterly.
The tracks are often described as " parabolic," though in the majority of
cases " hyperbolic " would be more accurate. The point of recurve is of
great importance for the forecasting of the subsequent path. This point
is often the seat of the maximum energy of the disturbance.
Reduction, as applied to meteorological observations, generally means
the substitution for the values directly observed of others which are computed
therefrom. The process of reduction generally eliminates the influence of
one or more factors that are not at the moment a subject of study. An
illustration of the process is given under REDUCTION TO SEA LEVEL.
Reduction to Sea Level.—Both temperature and pressure are " reduced
to sea level " before they are plotted on charts. To reduce mean temperature
to sea level 1° F. is added for each 300 ft. in the elevation of the station;
1° A. for 165 m. ; other rates are used for maximum temperature and
minimum temperature (see " Computer's Handbook", Introduction, p. 13).
This reduction is regarded as necessary in forming maps of ISOTHERMS of
regions with a considerable range of level, otherwise the isotherms simply
reproduce the contours; but it reduces the practical utility of the maps
because the addition of ten or twelve degrees to the temperature actually
observed gives an entirely false idea of the actual state of things in the locality
represented.
The same objection does not apply to the reduction of pressure to sea
level because the human organism has no such separate perception of pressure
as it has of temperature.
The reduction of pressure to sea level is carried out in accordance with
the general rule for the relation of difference of pressure to difference of
height. This goes according to the equation
h - h0 = k T (Iog10 p0 - logw p)
where h, p, h0, p0 are corresponding values of height and pressure, T is the
mean temperature of the air columns and k a constant which is numerically
equal to 67 • 4 when the height is to be given in metres, or to 221 • 1 when the
height is to be given in feet. This equation is derived from the direct
expression of the relation of pressure and height
g p dh = — dp.
£90565)
F2
REFLECTION
164
The details of the technique of reducing pressure to mean sea level will
be found in "Tables Meteorologiques Internationales," Paris, 1890, pp. B32-B52
and pp. 182-227 (Tables VII and VIII).
The humidity of the air makes very little difference to the computation
of height in our latitudes where temperature and moisture do not reach
tropical figures. The best way for allowing approximately for humidity,
which diminishes the density under standard conditions, is to regard the
temperature as increased by one tenth of a degree for each millibar of watervapour pressure in the atmosphere.
The vertical gradient of temperature varies according to the locality
and the season, but in the reduction of temperature to sea level it is usually
assumed to be at the rate of 0-5° C. per 100 m. or 1° F. per 300 ft. The
International Meteorological Conference at their meeting at Innsbruck in
1905 recommended that these values should be employed in the reduction
of pressure to mean sea level but the Conference at Utrecht in 1923 agreed
that in special circumstances other methods of reduction should be permitted.
Reflection.—The reflection of light and of radiant heat is an important
factor in the physics of the atmosphere. Reflection may be regular like
the reflection from a smooth sheet of water or diffuse like the reflection
from a sheet of white paper. The intensity of regularly reflected light
depends on the angle of incidence. Of the light which comes from the sun
a large proportion is reflected by the clouds. Some of this light may have
passed through two or three drops before reflection. Aldrich* has measured
the reflecting power of clouds and concludes that a cloud returns 78 per cent,
of the incident radiation, and that the amount of reflection is practically
independent of the angle of incidence.
In some cases continuous refraction of light produces an effect equivalent
to reflection. This is the explanation of MIRAGE.
Refraction.—The name applied to the bending to which rays of light
are subjected in passing from one medium to another of different optical
density. It plays an important part in many optical phenomena in the
atmosphere ; MIRAGE, HALOS, and RAINBOWS are refraction phenomena, the
colours of the two latter being due to the fact that rays of different colours
suffer a different amount of bending. Another refraction effect is that the
apparent ALTITUDE of a heavenly body is greater than its real altitude because
the rays of light entering the atmosphere are passing from a less dense to a
more dense medium, and their final direction is nearer the vertical than their
original direction.
Owing to refraction the setting sun is still seen above the horizon when
its geometrical position is below that level. The rays from the lower limb of
the sun are more refracted than those from the upper so that the sun may
appear to be somewhat flattened. When the stratification of the air is such
that there are rapid changes of density in the vertical, the sun may appear
much distorted and in some cases it can fade out of sight without apparently
reaching the horizon.
Under similar circumstances objects which are normally invisible may
appear above the horizon. For instance the coast of France has been seen
occasionally from Hastings.
Registering Balloon.—A small balloon, usually free, carrying a light
meteorograph recording pressure, temperature, and humidity of the upper
air, etc. See SOUNDING BALLOON.
Regression Equation.—An approximate relation, generally linear, con­
necting two or more quantities, derived from the CORRELATION COEFFICIENT.
* Washington D.C., Smithson. misc. Coll., 69, No. 10, 1919.
165
REVERSAL
Relative Humidity.—The ratio of the actual amount of water vapour
in a given volume of air to the amount which would be present were the air
saturated at the same temperature, expressed as a percentage, is termed the
relative humidity, or for the sake of shortness, simply " humidity."
Observations of relative humidity may be made directly with a hair
hygrometer, but it is more usual to compute the value from readings of dryand wet-bulb thermometers with the aid of tables. When great accuracy is
required an aspirated psychrometer is used. Continuous records are obtained
by means of hair-hygrographs or from thermographs arranged to record the
dry-bulb and wet-bulb temperatures.
In the British Isles relative humidity exhibits a large diurnal variation,
especially in the spring and summer months and in inland situations. High
values, often reaching 100 per cent., occur in the early morning hours, and
low values at the time of maximum temperature in the afternoon. The
seasonal range of mean relative humidity is comparatively small. See
AQUEOUS VAPOUR.
Reseau Mondial.—An annual publication of the Meteorological Office,
Air Ministry, issued under the auspices of the International Meteorological
Committee. It was recognized early in the present century that the next
important step in the progress of international meteorology, after the
publication of normal values for pressure, temperature and rainfall for the
world, was the compilation year by year of monthly means of pressure,
temperature and rainfall at stations in all parts of the world, organised to
secure the uniformity of practice necessary for the purposes of comparison.
The Committee appointed a commission, the International Commission
for the Reseau Mondial, to foster the enterprise and the publication of the
data commenced in 1917 with the volume for 1911. Since then the volumes
for 1910 and 1912 onwards have been published, that for 1931 being issued in
1938. The data are arranged on the basis of two stations for each ten-degree
square of latitude and longitude, and in the volumes for 1910 to 1920 only
land stations were included. In the volumes for 1921 onwards marine data
are also included. The information published for each station includes mean
pressure, mean and absolute maximum and minimum temperatures, the
observed or deduced mean of 24 hourly temperature readings and the total
rainfall, together with the differences from normal for all three elements.
Reshabar or rrashaba.—A name meaning " black wind " given to a
strong, very gusty, north-easterly wind which blows down certain
mountain ranges in southern Kurdistan. It is dry, comparatively hot in
summer and cold in winter.
Residual.—The difference between an individual observation and the
mean of a series, or the difference between an individual observation and
the value derived from the adopted values of the constants which have been
obtained by a discussion of the observations. Thus an observed quantity /
mav be known to be a function of variables x, y, z, and constants a, b, c,
of the form of ax + by + cz — I. If a number of observed values of I are
given for known values of x, y and z, there will be n equations to determine
the 3 constants a, b, and c. The equation will not in general be accurately
satisfied for any one observation, and the value of I — (ax + by + cz) is
called the residual.
Resultant.—The sum of a number of directed quantities or vectors.
COMPONENT and VECTOR.
See
Reversal.—A change of more than 90° in direction between the surface
wind and the wind in the upper air. A reversal may take place near the
ground or at any height up to 15,000 ft. or more. Reversals are common
phenomena over coasts when land and sea breezes are blowing in directions
greatly different from those that the general distribution of pressure would
REVOLVING STORM
166
dictate. The most permanent case of a reversal is over the trade wind, where
in the North Atlantic the north-easterly surface current is replaced by a
south-westerly one in the upper air. A spectacular example of this is afforded
when an eruption of the Soufriere in St. Vincent occurs, for then the dust,
carried by the upper current, falls in Barbados, though it lies 100 miles to
windward of St. Vincent.
Revolving Storm.—A term synonymous with tropical CYCLONE or HURRICANE.
See also LAW OF STORMS.
Ridge.—An extension of an ANTICYCLONE or high-pressure area shown
on a weather chart, corresponding with a ridge running out from the side
of a mountain. It is the opposite to a trough of low pressure.
Rime.—Soft Rime.—Deposits of white rough ice crystals that grow out to
windward of exposed objects when frost and wet fog occur together. The
minute supercooled drops of water, of which the fog is composed, freeze
when they come in contact with a solid object. Hoar-frost, on the other hand,
is formed by the direct freezing of water vapour, generally upon objects cooled
by radiation under a clear sky; the presence of fog, in so far as it checks
such radiation, tends to hinder the formation of hoar-frost.
Hard Rime.—Masses of ice deposited chiefly on vertical surfaces as described
for soft rime, but they are formed in " wet air " or supercooled drizzle at
temperatures below the freezing point and thus have a structure analogous
to that of small hail.
Roaring Forties.—A nautical expression used to denote the prevailing
westerly winds of temperate latitudes (below 40° S.) in the oceans of the
southern hemisphere.
Roll Cumulus.—Banks of cloud arranged in long parallel rolls with clear
spaces in between. A form of STRATOCUMULUS. See CLOUDS.
Rossby Diagram.—An aerological base diagram designed by C. G. Rossby
in which the potential temperature of the air on a logarithmic scale forms the
ordinate and the water content of the air in grams per kilogram (mixing ratio)
forms the abscissa. Equivalent potential temperature is given by sloping
lines ; this is the temperature which any parcel of air would assume if all the
water vapour which it contains were condensed at constant pressure and the
latent heat set free used to raise the air temperature. POTENTIAL TEMPERATURE
is the temperature which a parcel of air would assume if it were brought to
standard pressure (1000 mb.) under adiabatic conditions. Thus equivalent
potential temperature is that which the air would assume if all the water
vapour were condensed and the dry air then brought adiabatically to standard
pressure. Sloping curved lines on the Rossby diagram give the corresponding
constant equivalent potential temperatures and the isotherms and isobars of
dry air.
Both equivalent potential temperature and water vapour content are
conservative properties of an AIR MASS, i.e. they change little from day to day
and are therefore useful for identifying air masses in upper air soundings.
Rossby has shown that if the equivalent potential temperature decreases
with height the air is convectively unstable.
Run-off.—The portion of the rainfall over a DRAINAGE AREA which is
discharged from that area in the form of a stream or streams.
St. Elmo's Fire.—A brush-like discharge of electricity sometimes seen on
the masts and yards of ships at sea during stormy weather ; it is also seen
on mountains on projecting objects. It may be imitated by bringing a sharp
pointed object, such as a needle, near a charged Leyden jar. According to
Trabert's observations on the Sonnblick the character of St. Elmo's fire
changes with the sign of the electricity which is being discharged into the air.
The negative " fire " is concentrated so that an object like a mast is completely
enveloped in fire. The positive fire takes the form of streamers some three or
four inches long.
St. Luke's Summer.—A period of fine weather which is popularly supposed
to occur about the time of St. Luke's day, October 18.
167
SCIROCCO
St. Martin's Summer.—A period of fine weather which is popularly supposed
to occur about the time of St. Martin's day, November 11.
Salinity.—The salinity of a natural water, such as sea-water, is usually
expressed in parts per thousand. Thus a salinity of 35 per mille (written
35%0) indicates that there are 35 Ibs. of salt in 1,000 Ibs. of sea water. Since
the direct determination of total dissolved solids cannot be effected with
accuracy, salinity is derived in practice by applying factors to the halidecontent, which can be exactly estimated, or to the specific gravity. The
value above given is a rough average for surface ocean water.
Sand-pillar.—See DUST-DEVIL.
Sandstorm.—Sandstorms occur when a strong wind carries relatively
coarse sand through the air near the ground. A sandstorm rarely extends to
a greater height than 50-100 ft. above the ground, though fine dust particles
may be carried to greater heights, and the sand particles are not carried to
appreciable distances from their source.
There is a marked distinction between sandstorms and duststorms. See
DUSTSTORM.
Sastrugi.—(pi. : from the Russian) : Irregularities or wave formations
caused by persistent winds on a snow surface. They vary in size according
to the force and duration of the wind, and the state of the snow surface in
which they are formed.
Saturation.—A given volume of ordinary air which is exposed to a plane
surface of water or ice has for a given temperature a definite saturation
pressure of water vapour (see AQUEOUS VAPOUR) ; this saturation pressure
increases rapidly with increasing temperature. A fall of temperature would
lead to CONDENSATION of some of this water vapour, while a rise of temperature
would make the air unsaturated and therefore able to take up more water
vapour. It may be noted that water can under certain conditions remain
unfrozen at temperatures below 0° C. (32° F.) and the saturation pressure
relative to a surface of such " supercooled " water is greater than that relative
to ice at the same temperature.
Saturation Deficit.—See AQUEOUS VAPOUR.
Scalar.—A quantity which is completely specified by a number as opposed
to directed quantities such as wind velocity, which require for their complete
specification both a number or magnitude (the wind speed) and a direction.
Examples of scalar quantities in meteorology are temperature, pressure and
humidity, but it must be noted that pressure gradient is not scalar but vectorial
as its complete specification demands a direction as well as a number giving
the magnitude. See VECTOR.
Scintillation or Twinkling of the Stars.—A rapid apparent variation in
brightness and in colour. Scintillation occurs only with stars comparatively
low down in the sky. The elevation up to which scintillation occurs has been
found to vary in the course of the year. According to a series of observations
made at Lyons the maximum elevation 50° occurs in March and September,
the minimum 22° in June.* Scintillation is certainly due to irregularities in
the density of the air : the irregularities are carried along by the wind but
how they are related to eddies and to wave motion in the atmosphere is not
known.
Scirocco.—A warm, southerly or south-easterly wind which blows in front
of a depression passing from west to east along the Mediterranean. As this
wind comes from the Sahara desert it reaches the north coast of Africa as a
hot, dry wind, its temperature having been further raised by the descent of
the air from the inland plateau to the coastal regions. In crossing the
Mediterranean evaporation occurs and the wind reaches Malta, Sicily and
Italy and other parts of the European coast as a warm, moist wind.
The word scirocco seems to be used in some Mediterranean regions for a
warm, southerly wind irrespective of whether it is moist or dry. Many of
the warm, dry winds which are called sciroccos are probably FOHN winds.
* MMorologie, Paris, 1927, p. 193
SCOTCH MIST
168
Scotch Mist.—In mountainous or hilly regions, dense clouds are often
adjacent to the ground, and precipitation may occur in the form of minute
waterdrops, the apparent effect being a combination of thick mist and heavy
drizzle. The upland character of the greater part of Scotland and the
consequent frequency of occurrence of the phenomenon in that country have
secured for it the appellation by which it is generally known in the British
Isles. In the uplands of the Devon-Cornwall peninsula the same phenomenon,
which is there of very frequent incidence, is known as " mizzle."
The base of a true rain cloud rarely exceeds about 7,000 ft. (2-1 Km.) in
elevation, and sometimes descends to within a few hundred feet of sea level,
so that Scotch mist may be experienced in comparatively low-lying regions.
Screen, Stevenson.—The standard housing for meteorological thermo­
meters designed by Thomas Stevenson, C.E. It consists of a wooden cupboard,
with hinged door, mounted on a steel or timber stand, so that its base is
about 3 ft. 6 in. above the ground, the whole painted white. Indirect venti­
lation is provided through the bottom, double roof and louvered sides, and
thermometers placed within it give a close approximation to the true air
temperature, undisturbed by the effects of direct solar or terrestrial radiation.
The " ordinary " pattern accommodates the wet- and dry-bulb, maximum
and minimum thermometers. In the " large " pattern additional accom­
modation is provided for a thermograph and hygrograph.
Scud.—A word used by sailors to describe ragged fragments of cloud
drifting rapidly in a strong wind, often underneath rain clouds. The meteoro­
logical term is fractostratus. See CLOUDS.
Sea Breeze.—See LAND AND SEA BREEZES.
Sea Disturbance.—The scale of sea disturbance is as follows :—
Description of sea
Scale
Calm—glassy.
0
Rippled.
1
Smooth.
2
Slight.
3
Moderate.
4
Rough.
5
Very rough.
6
High.
7
Very high.
8
Phenomenal.*
9
Sea Level.—The "level" surface which a stagnant sea would assume in
the absence of waves, swell and tides. The surface is such that the normal
drawn to it at any point lies in the direction of the resultant of the accelerations
due to gravity and to the centrifugal force of the earth's rotation. The surface
is an oblate spheroid in which the short axis coincides with the axis of the
earth.
Owing to waves, swell and tides, the actual level of the sea is constantly
changing, and it is therefore usual to denote its mean position at any place
by the .expression " mean sea level." The amplitude of the fluctuation of
sea level from its mean position varies considerably from time to time and
from place to place, but it is believed that the mean position, or mean sea
level, remains practically constant at any place.
Mean sea level is generally used as the standard to which the contour
heights of the topography of the ground are referred.
The contours given on the Ordnance Survey maps of England have
hitherto been referred to the supposed position of the mean sea level at
Liverpool, but recently the standard has been changed to the ascertained
position of mean sea level at Newlyn in Cornwall. The Newlyn datum is
0-13 ft. below the Liverpool datum. Newlyn is more suited to the purpose
since it is situated on the edge of the Atlantic Ocean and is not subject to
sources of disturbance to which a place on a tidal river on an enclosed sea
* As might exist at the centre of a hurricane.
169
SEASONS
is liable. Contours on the Ordnance Survey maps of Ireland are referred
to an assumed mean sea level at Dublin, which has been estimated to be
about 8 ft. below that of the " level " surface passing through the corres­
ponding datum at Liverpool. For definitions of normal zero in the various
national surveys of Europe see Roseau Mondial 1914, pp. x-xi. See also
LEVEL.
Sea Temperature.—The normal method of measuring sea temperature is
to draw water in a bucket from the ship's side, forward of all ejection
pipes and to read the temperature of the sample with a specially designed
thermometer. The reading obtained is the mean temperature of the surface
layer of the sea to the depth of about a foot, and is known as the sea surface
temperature. No method has yet been devised for obtaining the temperature
of the actual surface of the water. The mean annual surface temperature
exceeds 80° F. over a broad belt of the equatorial region, and is somewhat
less than 30° F. in the polar regions. The run of the isotherms varies in the
two hemispheres and in the different oceans. The seasonal range of tem­
perature is of the order of 10° F. in both polar and equatorial regions, and is
greater in middle latitudes, where for the most part it lies between 10° F.
and 30° F. The greatest range, 50° F. or more, is found in small areas,
extending to the coast of the western North Atlantic and western North
Pacific Oceans. The diurnal variation of sea surface temperature is very
small, 1° F. or less.
Seasons.—The seasons in this country according to the " farmer's year '
are :—autumn : September, October, November ; winter : December,
January, February; spring: March, April, May; summer: June, July,
August.
The idea of four seasons in agriculture appropriate to these islands, the
winter for tilling, the spring for sowing and early growth, the summer for
maturing and harvesting, and the autumn for clearing and preparing, depend
upon the peculiarity of our climate. Where the land is ice-bound in winter
or rainless in summer another distribution has to be made.
Between the tropics there is nothing that can properly be called summer
and winter ; the seasons depend upon the weather and rainfall, and not upon
the position of the sun, and the periods of growth are adjusted accordingly.
In India, or the north-western part of it, the divisions of the year are the
cold weather, the hot weather, and the rains. At the polar margins the
change from winter to summer and vice versa is so sudden that there the
transition seasons, spring and autumn, largely disappear.
The selection of months to represent the seasons according to the farmer's
year is guided by the consideration that each season shall comprise three
months. This uniformity in the length of the seasons, leads at times to para­
doxical results. Thus, in a particular year, the warmest week may occur late
in May or early in September, i.e. in spring or autumn and not in summer
as defined above ; or again, the coldest week may occur late in November
or early in March, i.e. in autumn or spring, and not in winter as defined
above. From the point of view of weather, we have in this country about
five months of moderate winter weather between October and April, and four
months of summer weather from the middle of May to the middle of September,
a short spring and a short autumn ; but the seasonal variations are not nearly
so large here as they are in continental countries, and the change from winter
to summer and vice versa is much less abrupt. Temperature, rainfall and
sunshine data for individual stations and for districts are published in the
Weekly Weather Report of the Meteorological Office and enable one to follow
the course of the seasons in the British Isles. In the " Book of Normals of
Meteorological Elements for the British Isles," Section II, are given normal
values for each week of the year of mean temperature, rainfall and sunshine
for each of twelve districts into which the British Isles have been divided.
For information regarding the progress of rainfall from month to month in a
normal year, the reader may consult in addition the " Rainfall Atlas of the
(90565)
F*
SECONDARY COLD FRONT
170
British Isles," published by the Royal Meteorological Society. Some
interesting particulars of the temperature of the several seasons in the British
Isles are also given in " Temperature Tables of the British Isles," M.O.
154, 1902, which includes diagrams showing the average daily maximum,
minimum and mean temperatures, and the highest and lowest values
respectively recorded on each day throughout the year in the period
1871-1900 at four observatories : Aberdeen, Valentia, Falmouth and Kew.
The curves of mean temperature variation from day to day show numerous
irregularities, but in their general form show a more or less progressive
increase and decrease between a minimum in December or January and a
maximum in July or August.
Secondary Cold Front.—A COLD FRONT in polar air, following the first
cold front. If the polar current is unstable, there is sometimes an irregular
series of secondary cold fronts of limited length, accompanied by heavy
squalls and sometimes by thunder. The fall of temperature is often due
mainly to the precipitation, and after the squall passes, the temperature
sometimes recovers almost to its previous value.
Secondary Depression or " Secondary."—The isobars around a DEPRES­
SION are frequently not quite symmetrical, they sometimes show bulges or
distortions, which are accompanied by marked deflections in the general
circulation of the wind in the depression; such distortions are called
secondaries ; they may appear merely as sinuosities in the isobars, but at
other times they enclose separate centres of low pressure and show separate
wind circulations from that of the parent depression. In short, a secondary
is a small area of low pressure accompanying a larger primary depression.
A secondary depression generally travels with its primary, but in addition
the secondary, as a rule in the northern hemisphere, rotates about the primary
in a counter-clockwise direction. On one synoptic chart the secondary may
be shown on the western side of the primary depression while on the following
day it may be on the eastern side of the depression, having as it were rolled
round the southern side of the primary. Frequently the secondary becomes
deeper while the primary becomes less intense ; when such is the case the
primary and the secondary tend to form a complex dumb-bell shaped
depression, the two centres of low pressure revolving round each other in a
counter-clockwise direction. Ultimately the secondary absorbs the parent
depression, the two coalescing and forming one. When the secondary appears
first as a sinuosity it often gives rise to precipitation of a more or less con­
tinuous type. As it develops and gradually attains a wind circulation about
its own centre the weather tends to resemble that of an ordinary depression.
Sometimes the secondary becomes so intense and deep that it gives rise to
very violent and destructive gales. On the other hand, a secondary may
have, as frequently happens in summer, a very feeble wind circulation and
such a secondary sometimes gives rise to thunderstorms. The weather,
however, associated with these systems is very capricious.
The wind circulation in a secondary obeys BUYS BALLOT'S LAW, and
thus in the northern hemisphere, the wind circulates around it in a counter­
clockwise direction. The region between the primary depression and the
secondary is generally one of light and variable airs, the strongest winds
beinff found on the side away from the main depression.
Many new depressions with warm sectors first appear as secondaries.
Secular Trend.—In statistics, a persistent tendency for a variate to increase
or decrease with the passage of time, apart from irregular variations of
shorter period. In meteorology, examples are the increase of pressure and
temperature at St. Helena and the decrease of rainfall at Sierra Leone.*
Such changes obviously cannot continue indefinitely, and they are probably
parts of oscillations of greater length than the available statistics. Secular
trends can be determined by smoothing the data sufficiently, or by correlating
with " time."___________________________
* See London, Geophys. Mem., No. 33, 1926.
to face Page /7°
Plate 17.
HIGH
MARCH 8, I92a,7h.
SECONDARY DEPRESSION
171
SHADOW OF THE EARTH
Seiche.—The name given to the quasi-tides which were first observed to
occur in the Lake of Geneva. It was known for some three centuries that the
water of this lake is apt to rise and fall, sometimes by a few inches, occasionally
by several feet. The phenomena were first investigated in detail by Professor
Forel of Lausanne, and it was found that oscillations of the water were to
be observed every day, with amplitudes varying from a millimetre to about
a metre, and with periods between 20 and 40 minutes.
The phenomena are described and discussed in considerable detail in
G. H. Darwin's " The Tides," where it is shown that the oscillations in the
level of the surface are to be explained as long waves in relatively shallow
water. Anything which heaps up the water at one end of the lake, and
then ceases to act, must tend to produce an oscillation of the whole. Among
the important causes of seiches are winds, small earthquakes which tilt the
bed of the lake, and probably the atmospheric oscillations which are recorded
as waves by the microbarograph. Some of the Japanese lakes show marked
periods round about eight minutes, in agreement with the periods of greatest
frequency in the microbarograph in fine weather.
Another type of seiche, called a temperature seiche, was discovered by
Watson and Wedderburn in some of the Scottish lakes. There is often a
stratum of abrupt change of temperature below the surface of a lake, and
the observers mentioned noted waves in this layer of transition.
Seiches are to be found in all lakes, and analogous phenomena are to be
noted in landlocked areas of the sea. There is now a considerable literature
dealing with the subject, and special reference should be made to papers
in the Transactions of the Royal Society of Edinburgh from 1905 onwards,
by Chrystal and his pupils White, Watson and Wedderburn.
Seismograph.—An earthquake recorder or instrument for automatically
recording the tremors of the earth. A seismological observatory requires
three seismographs to show the vertical and two horizontal components of
the motion of the ground.
Seisms.—Oscillations of the crust of the earth. See EARTHQUAKES,
MlCROSEISMS.
Seistan Wind.—A strong northerly wind which blows in summer in the
province of Seistan, in eastern Persia. It continues for about four months,
and is, therefore, known as the "wind of 120 days." It sometimes reaches
hurricane force.
Serein.—Fine rain falling from an apparently clear sky. It is very rare.
Shade Temperature.—The temperature of the air indicated by a ther­
mometer sheltered from precipitation, from the direct rays of the sun and
from heat radiation from the ground and neighbouring objects, and around
which air circulates freely. A standard shelter such as the Stevenson Screen
is intended to satisfy these conditions. Provided certain precautions are
taken the result is a close approximation to the actual temperature of the
air. See SCREEN.
Shadow of the Earth.—The shadow of the solid earth is the region from
which direct sunshine is cut off by the intervention of the globe. The height
of the shadow overhead is frequently made manifest by the changing illu­
mination of the clouds. A small cloud which is in sunshine and appears a
beautiful pink at one minute is in shadow the next and fades to a dull grey,
whilst higher clouds are still illuminated. At such a time the background of
the clouds is blue, for the air at greater heights is still illuminated by direct
sunshine.
The shadow thrown on the air itself can also be observed with ease. Soon
after sunset a dark segment is to be noticed in the eastern sky encroaching
on the " counterglow " the band of reddish tint which runs round the horizon.
This dark segment is the earth's shadow. As the boundary of the shadow
rises the upper edge of the counterglow remains almost stationary and both
are usually lost when the sun is about 4° below the horizon and the elevation
of the boundary of the shadow is about 10°.
(90565)
F*2
SHAMAL
172
Whilst the shadow is perceptible all the air to the east of the observer
is illuminated by light which has been scattered by the atmosphere, but the
air at great heights is illuminated also by direct sunshine. The shadow is
the part of the sky in which the additional light which comes to the observer
from such great heights is inappreciable. The boundary is sharp when the
line of sight is nearly parallel to the sun's rays but when the shadow climbs
the observer is looking obliquely across the sun's rays and the boundary
becomes indefinite and is lost.
When the sun is about 10° below the horizon, directly illuminated air
is to be seen only near the western horizon ; the upper limit of the illuminated
segment is the boundary of the earth's shadow. This boundary is sometimes
called the TWILIGHT ARCH. By the time the sun is 18° below the horizon,
if not before, the whole of the atmosphere within sight is in shadow, and
TWILIGHT has ended.
Shamal.—A north-westerly wind which blows in summer over the Mesopotamian plain. It is often strong during the daytime but decreases at night.
Shower.—In describing present or past weather, the following distinction
is made between the use of the terms " showers " and " occasional pre­
cipitation." In general, showers are of short duration and the fair periods
between the showers are usually characterised by definite clearance of the
sky. The clouds which give the showers are, therefore, isolated. The pre­
cipitation does not usually last more than fifteen minutes, although it may
sometimes last for half an hour or more. Typical showers occur in the W. or
NW. current of POLAR AIR behind a depression which has passed eastwards.
This current is warmed from below and becomes unstable, hence these showers
are called instability showers ; in the Beaufort notation they are termed
" passing showers " and are denoted by " p." Occasional precipitation, on
the other hand, usually lasts for a longer time than the showers and the
sky in the periods between the precipitation is usually cloudy or overcast.
Silver Thaw.—An expression of American origin. After a spell of severe
frost the sudden setting in of a warm, damp wind may lead to the formation
of ice on objects, which being still at a low temperature cause the moisture
to freeze upon them and give rise to a " silver thaw." See GLAZED FROST.
Simoom.—A hot, dry suffocating wind or whirlwind which occurs in
the deserts of Africa and Arabia. It usually, but not invariably, carries a
large quantity of sand. It does not usually last long, not more than about
20 minutes. It is most frequent in the spring and summer.
Sine Curve.—See HARMONIC ANALYSIS.
Single Observer Forecasting.—The method of forecasting used by isolated
observers when cut off from weather information from neighbouring areas.
Most weather forecasts prepared by Meteorological Services are based on
synoptic charts and are issued to the public by radio, telegraph, telephone,
etc., or through the press. These forecasts are often accompanied by synoptic
data from which the recipient can draw his own weather charts. When, as
sometimes happens for various reasons, this information is not available,
the " Single Observer " must rely on his own observations of cloud, wind,
temperature, pressure, etc., to tell him what weather to expect. The develop­
ment of the theory of air streams and their boundary " fronts " enables the
changes in the atmosphere to be more readily interpreted and provides the
links by which various weather maxims may now be joined together.
Although the forecasts issued by the single observer cannot compare in
accuracy with those based on synoptic charts, a certain amount of success
can be obtained for a few hours ahead, especially if the observer has lived
173
SNOW
long enough in one are to become familiar with local peculiarities. See also
WEATHER MAXIM and " The Fishery Barograph," (M.O.333), a note on the
use of the barograph in anticipating gales.
Sirocco.—See SCIROCCO.
Site.—In order to secure observations comparable with those at other
stations, the site of a meteorological STATION has to be carefully selected in
accordance with certain rules which are set out in the " Meteorological
Observer's Handbook." A rain-gauge requires a certain amount of protection
from the wind, but for the other outdoor instruments the more open the
site, the better. A compromise is usually effected. The latitude, longitude
and height of the ground on which the rain-gauge stands are used to define
the position of a station.
Skewness.—See MEDIAN.
Sky, State of the.—Fraction of the sky obscured by cloud on a scale of 0
(cloudless) to 10 (an entirely overcast sky in which no patches of blue sky
are visible). In the BEAUFORT NOTATION letters, b, blue sky whether with
clear or hazy atmosphere, c, cloudy, i.e. detached opening clouds, o, sky
overcast with one impervious cloud, g, gloom, and u, ugly, threatening
sky are also used in addition or alone to indicate the general appearance
of the sky. (See " Meteorological Observer's Handbook.")
Sleet.—Precipitation of snow and rain together or of melting snow and
rain. In some countries there is no special term for this form of precipitation.
In America the term is used in a different sense: Humphreys (in "Physics
of the Air ") defines it as " ice pellets, mere frozen raindrops or largely melted
snowflakes re-frozen—frozen during the fall of the precipitation through a
cold layer of air near the surface of the earth—that rattle when they strike a
window.''
Slide Rule, Pilot Balloon.—A slide rule designed to facilitate the rapid
computation of the components of the wind velocity from observations of
altitude and azimuth of a PILOT BALLOON. 'Provision is also made, in the most
recent type, for computing the heigh when the " tail " method of observation
is employed.
Sling Thermometer.—A thermometer mounted on a frame pivoted about
a handle so that it can be whirled in the hand, thus providing " ventilation."
If the bulb is shielded from direct solar radiation satisfactory readings of air
temperature can thus be obtained in a simple and inexpensive manner. A
pair of thermometers, dry- and wet-bulb, similarly used, constitute a " sling "
or " whirling " psychrometer. See also PSYCHROMETER.
Small Hail.—See HAIL.
Snow is precipitation in the form of ice crystals of feathery or needlelike
structure. The crystals may fall singly, or a large number of them may be
matted together in the form of large flakes.
The characteristic shape of the simplest ice crystal is a small flat triangular
or hexagonal plate, but most snow crystals show much more complicated
structures, ranging from needles to six-pointed stars with numerous spreading
branches. More than 2,000 forms have been photographed by W.A. Bentley
at Jericho, Vermont, Canada. The method of formation of such stars has
been described by ti. Lehmann, who has watched under the microscope the
development of star crystals of iodoform and other substances from a slightly
supersaturated solution of the substance.
According to Lehmann the first step in the process is the formation of
a minute flat crystal in the form of a regular hexagon. The crystal continues
to grow in that form until it is seen from the lighter colour of the solution
immediately bordering on it, that the supersaturation is now confined to the
parts of the liquid which are more distant from the crystal, while the solution
SNOW, DAY OF
174
in immediate contact with the crystal is only just saturated. Thereafter the
crystal grows more slowly and only as the more concentrated liquid finds
its way to the crystal by diffusion. The parts of the crystal nearest to the
concentrated liquid (that is, the corners of the hexagon) are those first affected
by this process of diffusion ; consequently from this stage the crystal begins
to develop pointed rays which grow outwards from the six angles of the
original hexagon. This process proceeds until the rays are so long as to allow
relatively concentrated solution to diffuse in between them; this in turn
induces sideways growths of spikes from the original arms of the star. If
now a weakening of the concentration of the outside solution occurs, then the
arms of the star will cease to grow outwards, but facets will tend to form at
their ends and the growth of the side spikes will continue. Alternations of
increasing and decreasing concentration will produce varied forms of crystal.
Wegener showed that symmetrical branching snow crystals might be
expected to form by a similar process in air below freezing point, having a
moisture content in excess of that corresponding to saturation with respect
to ice at the air temperature.
Recently our knowledge has been extended by Nakaya and various,
collaborators, who have found it possible to reproduce artificially the various
forms of simple ice crystals and star-shaped snow crystals observed in nature.
Nakaya found that delicate fern-like snow crystals were produced in his
apparatus with an air temperature of the order of — 20° C. and an abundant
supply of moisture. At much lower temperatures only irregular crystal
aggregates could be produced. (See London, Quart. J. R. met. Soc., 64, 1938,
pp. 619-24.)
A foot depth of freshly fallen snow is equivalent roughly to one inch of
rain, but the ratio varies considerably with the depth and texture of the snow.
After deposition progressive changes of texture and therefore of density occur.
For a full discussion of the snow and ice forms met with in nature, see
G. Seligman " Snow Structure and Ski Fields " (Macmillan, 1936).
Snow, day of.—In British climatology any period of 24 hours ending at
midnight G.M.T. upon which snow is observed to fall is regarded statistically
as a " day of snow."
Snowdrift.—When snow falls on a day of strong wind it does not readily
settle in open places, but will normally do so wherever there is a region
which is sheltered from the full force of the wind. Much of the dislocation
of traffic caused by heavy falls of snow is due to the " drifts " formed in this
way. Drifts may even be formed when no snow is falling, by the action of the
wind upon already fallen snow ; snow carried in this way is called drift-snow.
The word, therefore, has two meanings :—
(1) an accumulation of snow, forming drift;
(2) snow that drifts with the wind.
Two international symbols were adopted at Warsaw in 1935 to denote
drift snow, namely "h* to be used when the snow is drifting at ground level,
and -f-* to be used when the drifting is occurring at a high level so as to affect
vertical velocity.
Snow-gauge.—A device for the retention and measurement of snow. In
the Hellmann-Fuess snow-gauge the snow is caught in a receiver supported
on a balance, the displacement of which is continuously recorded, so that an
autographic record of snowfall (and of the fall of rain and hail also) is obtained.
Most snow-gauges are, however, merely rain-gauges fitted with jackets or
other devices to make them suitable for collecting solid precipitation, and for
melting it before taking the reading.
Snow Line.—The lower limit in altitude of the region of perpetual snow,
In high polar latitudes the snow line is at sea level; in northern Scandinavia,
it is at about 4,000 ft., in the Alps at about 8,500 ft., in the Himalayas at
175
SOLAR CONSTANT
about 15,000 ft. These figures are only approximate, as the height of the
snow line varies on the north and south sides of a mountain and from one
mountain to another in the same latitude or region. It has no direct relation
to the mean annual temperature, depending more on the summer temperature,
but many other factors exert an influence, such as amount of snow in winter,
prevailing winds, exposure and steepness of the slopes, etc.
Snow Lying.—This expression (International symbol O) is used for occa­
sions when one-half or more than one-half of the ground representative of the
station is covered with snow. The ground representative of the station is
defined as " the flat land easily visible from the station and not differing from
it in altitude by more than 100 ft." British statistics of snow lying refer
only to occasions when this state of affairs exists at the hour of morning
observation.
Snow Rollers.—Cylinders of snow, formed and rolled along by the wind.
Soft Hail.—See HAIL.
Soil Moisture.—A factor of obvious importance in relation to the growth
of plants, soil moisture is also of meteorological interest because it affects
the thermal diffusivity of the soil and, therefore, the rate at which heat can
be conducted upwards or downwards. In the instrument designed by
W.S.Rogers (see Annual Report of the East Mailing Research Station, 1935,p. Ill),
soil moisture is measured by determining the " capillary pull " of the soil.
This force is due mainly to the surface tension of the water film surrounding
the soil particles, and it varies with the amount of moisture in the soil, the
size of the soil particles and the degree of compactness of the soil. The
instrument consists of a water-filled porous pot connected with a pressure
gauge or manometer. The pot is buried to the required depth and the capillary
pull is indicated by the pressure gauge.
Solano.—An easterly wind which brings rain to the south-east coast of
Spain and in the Straits of Gibraltar.
Solar Constant.—The solar constant is the intensity of the RADIATION in the
solar beam in free space at the earth's mean distance from the sun. Obser­
vations of solar radiation have been made for many years on high mountains
in desert climates at stations organised by Dr. G. C. Abbot and his colleagues
of the Smithsonian Institution. In the fundamental observations the spectral
distribution of the intensity of radiation is determined for various altitudes
of the sun and the intensity outside the atmosphere is deduced by extra­
polation. The consistency of the observations has improved steadily since
1905, but not all of the vagaries of atmospheric absorption can be allowed
for, and even in 1934 the probable error of a single determination of the
solar constant had not been reduced below 1 per cent. Dr. Abbot has main­
tained that the observations indicate appreciable variations in the solar
constant. Thus he finds that the annual mean, expressed in terais of the usual
unit, the calorie per square centimetre per minute, was 1-933 in 1929 and
1 • 947 in 1934. The variations are associated with changes in the constitution
of the radiation. The energy of the long waves does not vary. It is in the
blue and violet, and especially in the ultra-violet, that fluctuations occur.
In other words, if Dr. Abbot's interpretation of the observations can be
accepted, the sun is a variable star which undergoes changes of colour. On
the other hand it must be mentioned that a detailed careful examination*
of the published statistics has led Dr. M. M. Paranjpe to the conclusion that
" there is no direct evidence that the solar constant is subject to variations."
The value of the constant may be taken as 1-94 cal./cm. 2/min. This is
equivalent to 135 mw./cm. a
* London, Quart. J.R. met. Soc., 64, 1938, p. 473
SOLAR DAY
176
Solar Day.—See EQUATION OF TIME, CALENDAR.
Solar-Radiation Thermometer.—See BLACK-BULB THERMOMETER.
Solarimeter.—An instrument designed by L. Gorczynski for determining
the intensity of solar radiation. It consists essentially of a sensitive thermopile
of low thermal capacity and low resistance connected in series with a millivoltmeter. According to the method of mounting, it may be used to determine
the intensity of the normal solar radiation or the vertical component of the
total radiation from sun and sky. If a recording millivoltmeter is substituted
for the direct-reading instrument, the combination is know as a " solarigraph."
Solstice.—The time of maximum or minimum declination of the sun,
when, for a few days, the altitude of the sun at noon shows no appreciable
change from day to day. The summer solstice for the northern hemisphere,
when the sun is farthest north of the equator, is about June 22, and the
winter solstice, when it is farthest south, is about December 22. After the
summer solstice the days get shorter until the winter solstice and vice versa.
Sounding Balloon.—For the purpose of measuring pressures and
temperatures, etc. in the upper air, a balloon is employed larger than a
pilot balloon, having a diameter (at ground level when inflated with hydrogen)
of about four feet. This balloon carries a light meteorograph attached to
a bamboo framework or " spider." In order that the meteorograph may
be some distance from the balloon, which is likely to become heated owing
to sunshine, a special launcher is employed ; this launcher gradually unwinds
a length of cotton thread as the balloon ascends, finally leaving the meteoro­
graph some hundred feet below the balloon. The balloon rises, expanding
as it goes, and ultimately bursts, probably having reached an altitude of over
12 miles. The remains of the balloon, together with the instrument, then
fall to the ground, the bamboo framework protecting the instrument from
mechanical injury. A label is attached asking the finder to return the
meteorograph. It occasionally happens that a balloon, which for some reason
does not burst, floats for may hours and is carried hundreds of miles before
falling to earth. The meteorograph designed by the late W. H. Dines,
weighs 2£ ounces, including the aluminium case, while the bamboo framework
weighs about If ounces. Owing to this light weight, no parachute or other
appliance, in addition to the bamboo framework, is needed to break the fall.
The meteorograph referred to is constructed as follows. The pressure
recorder operates by means of an aneroid box, the sides of which are
connected by arms with the scribing point and the record plate, thus an
expansion or contraction of the box results in a movement of the point
across the plate. The thermograph for recording temperature consists of
an invar rod and a thin strip of nickel silver, rigidly fastened together at
their lower ends, and connected to a lever by spring joints at their upper
ends. The expansion and contraction of the strip of nickel silver relative
to the nearly invariant invar rod, cause a vertical movement of the scribing
point relative to the plate. During calibration, definite marks at known
pressures and temperatures are made on the actual record plate, and the
final trace can be compared with these marks. The actual trace on a silvered
plate is very durable, and even if the meteorograph lies out in the open for
months before being found, the record is unlikely to be damaged.
A modification of the above arrangements is required for temperature
records over the sea. If a meteorograph were carried by a single
balloon, the instrument would be lost owing to sinking should the balloon
burst. It is necessary, therefore, to employ two balloons of a larger size
than those mentioned above. The two balloons are flown tandem fashion,
and the top balloon is inflated to have a lift of about 8 Ibs., while the lower
balloon has a lift of some 2 Ibs. A sea-anchor float is carried about 30 ft.
SOUNDING BALLOON
177
below the meteorograph. Upon release the balloons and meteorograph etc.
ascend, the upper balloon ultimately bursts (being more fully inflated than the
lower balloon) and the lower balloon allows the apparatus to descend to the
water, where, relieved of the weight of the sea-anchor, the small balloon is
capable of supporting the meteorograph and keeping it clear of the surface of
the sea. The floating balloon is easily observed and the meteorograph recovered.
Instead of relying upon the upper balloon to burst, a special dropper may be
used to release the top balloon at a given altitude. A similar system of two
balloons is employed when heavier instruments are used. (See RADIO-SONDES.)
CURVES SHOWING CHANGE OF TEMPERATURE WITH HEIGHT ABOVE SEA-LEVEL
OBTAINED FROM BALLON-50NDE ASCENTS 1907-8.
?0MANCHESTER
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TEMPERATIIRE IN. DECREES (X
260
230
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260
TEMPCRATURE ABSOLUTE
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300
FIG. 26.—The separate curves represent the relation between temperature and height
in miles or kilometres in the atmosphere. The numbers marking the separate curves indicate
the date of ascent at the various stations as shown in the tabular columns. The difference
of height at which the isothermal layer is reached, and the difference of its temperature for
different days or for different localities, is also shown on the diagram by the courses of the
lines.
The results of numerous ascents are given in the diagram (Fig. 26) which
exhibits graphs of temperature and height for 45 soundings.* It will be
noticed that, with one or two exceptions, the balloons reached a position
(approximately six or seven miles) above which the temperature ceased to
fall, yet the range of temperature at that level is large. See STRATOSPHERE,
TROPOSPHERE.
The great convenience of the radio-sonde which signals its readings as
they are made has led to the abandonment of the sounding balloon (1949).
* See also London, Geophys. Mem.. No. 5, 1913.
ascents. By E. Gold.
The international kite and balloon
SOUTHERLY BURSTER
178
Normal Pressure for the Several Months at Various Heights over south-east
England.
(Computed from Sea-level Pressures at Kew, and the Temperatures in Table IV, 2,
Computer's Handbook, II. § 2. p. 55.)
Septem,ber
November
December
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February
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October
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15
116
116
116
118
Pressure in millibars.
121 123 125 125
14
13
12
11
10
136
159
187
218
255
135
159
186
217
254
136
159
186
217
254
138
162
189
221
259
142
165
193
226
264
144
168
196
229
267
146
170
199
232
270
9
8
7
6
5
297
346
401
463
532
297
346
401
462
532
297
346
400
462
531
302
350
405
467
535
307
356
410
472
540
311
360
415
476
544
314
363
417
478
546
124
122
119
117
9
146
170
198
232
270
145
169
198
231
269
142
166
194
227
265
139
163
191
223
260
137
160
188
220
256
11
11
13
15
16
313
362
416
478
546
312
361
416
477
545
308
356
411
472
540
303
351
406
468
536
299
347
402
464
533
17
17
17
16
15
4
610 609 608 612 616 620 621 621 620 616 612 610 12
3
696 695 693 697 701 704 705 705 704 700 697 695 13
2
792 790 789 791 795 797 798 797 798 794 792 790
9
1
899 897 895 897 900 901 902 900 902 898 897 896
7
Gd.
1018 1016 1014 1014 1016 1016 1016 1015 1017 1014 1014 1014
4
At all heights above the ground level the maximum pressure occurs in the hottest months.
The seasonal variation in the normal pressure at sea level is small.
Table of Results obtained with Sounding Balloons in the United Kingdom.
Normal Temperature at different levels in the atmosphere up to twelve kilometres for the
___________________several months of the year.
_____________
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55
61
67
73
78
83
89
221
22
26
33
41
48
55
62
68
74
79
83
89
221
21
26
33
41
47
54
61
67
73
78
81
86
V
i—>
•a
i—Pi
I
<
o>
a
go
IH
<U
1
£
0)
O
<L>
Q
rt
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•A.
"A.
"A.
"A.
219
20
24
31
38
45
51
58
64
70
75
79
83
218
19
23
28
35
41
49
55
61
67
72
75
80
217
18
21
25
32
38
45
52
58
64
69
72
77
220
19
23
28
35
42
49
55
62
68
73
77
82
Taken from London, Geophys. Mem., No. 2 (W. H. Dines), 1911.
Southerly Burster.—A name given in south and south-east Australia to
a sudden change of wind usually from a north-westerly direction to a southerly
direction, which is accompanied by a sudden fall in temperature. This
change of direction occurs behind a cold front, and if the rise of pressure is
considerable the southerly wind is violent. The arrival of the southerly wind
is usually marked by a long crescent-shaped roll of cloud. The temperature
sometimes falls as much as 30° F. or 40° F. in half an hour. These storms are
sometimes accompanied by thunder and lightning. They are similar to the
PAMPEROS of South America and the LINE-SQUALLS of our latitudes. They
are most prevalent from October to March.
Spring.—See SEASONS.
179
STANDARD GRAVITY
Squall.—A strong wind that rises suddenly, lasts for some minutes, and
dies comparatively suddenly away. It is frequently, but not necessarily,
associated with a temporary change in direction. The most important
squalls in the British Isles, especially from the view-point of aviation, are
the LINE-SQUALLS in which the heavy increase in velocity is accompanied
by a veer of some 3 to 8 points and with which are associated the long arch
of low black cloud (which gives the squalls their name) and rain or hail with a
sudden drop of temperature. Squalls are to be distinguished from gusts, the
constant fluctuations which occur in any wind due to local obstructions and
surface friction.
Squall Line.—The name originally given to what is now known as the
COLD FRONT of a depression. It marks the line along which cold air is
undercutting warm air. Its passage is marked usually by squalls (hence
the older name), and, in such countries as the British Isles, it is often associated
with heavy cloud and rain or hail. With its passage the barometer rises
abruptly and there is a sudden or rapid fall of temperature. See LINESQUALL.
Stability.—A state of steadiness not upset by small deplacements or
disturbances. See INSTABILITY.
Standard. —A prescribed measure or scale of any kind, such as a weight,
length, volume, etc., which is used as a unit or scale of reference. The legal
magnitude of a unit of measure or weight.
Standard Atmosphere.—The International Standard Atmosphere which is
used as the basis of graduation of altimeters assumes at mean sea level a
temperature of 15° C., a pressure of 760 mm. (1013-2 mb.), and a lapse
rate of 6-5°C./Km. from sea level up to 11 Km., above which the
temperature is assumed constant at — 56-5° C.
For purposes such as aircraft design for which this International Standard
is not sufficient, the standard atmospheric temperatures denned by the
table on p. 179 (issued by the Ministry of Aircraft Production) may be assumed.
Tropical Summer Standard. —These temperatures are based on statistics
of the Indian Meteorological Department, 1935-1937, and are representative
of tropical conditions and those prevailing in the " Near East ".
Temperate Summer Standard. —These temperatures are based on observa­
tions at Kew and Sealand over the period 1921-1935 and on records of the
Duxford Meteorological Flight. The arbitrary maximum has been chosen so
that European conditions are covered for design purposes.
Sub-arctic Winter Standard. —These temperatures are based on observations
at Petropavlovsk (lat. 59° 41' N.) 1902-1909, Abisko (lat. 68° 21' N.)
1907-1929 and Obdorsk (lat. 66° 31' N.) 1936.
Standard Deviation.— If n observations of a quantity are #lf #2, . . #n,
and their mean or average is x, defined by nx = sum of the n individual
observations, the standard deviation a is defined by the equation : —
* 0S = (*1 ~ *) 2 + (*2 - *) 2 + • • + (*n - *) 2
When the observations are distributed according to the NORMAL LAW OF
ERRORS, there are certain theoretical advantages in using (n — 1) a2 instead
of na2 on the left hand side of the equation above, but in practice the difference
between the two formulae is small.
The square of the standard deviation is called the variance.
Standard Error.—The STANDARD DEVIATION is usually calculated on the
assumption that the mean of the sample of observations available is the true
mean of the element. This is not generally correct, the true mean being
unknown. An approximate allowance for this source of error may be made
by dividing the sum of squares of departures from mean by n — 1 instead of
by n, giving as the most probable value of the standard deviation about the
true mean
(x, - *)«
n -\
This is often termed the " standard error " of the sample.
Standard Gravity.—See GRAVITY.
STANDARD TEMPERATURE
180
Standard Temperature.—The temperature at which a mercurial barometer
reads true millibars at sea level in lat. 45°. It is clearly desirable that in
any barometer the standard temperature should not differ much from the
average temperature of the place where it is used, and a value of 285° A.
is aimed at for barometers used in this country. This is secured if the
barometer has no index error and the scale is engraved in accordance with
the relationship 1 barometer millibar = -0295975 barometer inch.
Standard Time.—Time referred to the mean time of a specified meridian.
The meridian of Greenwich is the standard for western Europe. The
standard meridian for other countries is generally chosen, so that it differs
from Greenwich by an exact number of hours or half-hours. (See also ZONE
TIME.)
State of the Ground.—See GROUND, STATE OF.
State of the Sky.—See SKY, STATE OF THE.
Station.—In meteorology, the word signifies a fixed place where regular
meteorological observations are made. Stations vary greatly from one
another in equipment and in the character and extent of the observations
made, but at all stations which report to a central Meteorological Office the
observations are made on a uniform plan and according to a set time-table,
so that the results or summaries of them can be published in an orderly
way, rendering comparisons easy and reliable.
The number of stations in the British Isles is about 5,500 ; by far the
majority of these are maintained by voluntary observers who in most cases
report measurements of rainfall only.
The International Congress of Meteorologists which assembled at Vienna
in 1873, adopted a classification of stations, based on the scheme of obser­
vations to be taken, as follows :—
(1) First-order stations of the International Classification. Normal
meteorological observatories : at which continuous records, or hourly
readings, of pressure, temperature, wind, sunshine, and rain, with eye
observations at fixed hours of the amount, form, and motion of clouds
and notes on the weather, are taken.
(2) Second-order stations of the International Classification.
Normal climatological stations : at which are recorded daily, at two
fixed hours at least, observations of pressure, temperature (dry- and
wet-bulb), wind, cloud and weather, with the daily maxima and minima
of temperature, the daily rainfall and remarks on the weather. At
some stations the duration of bright sunshine is also registered.
(3) Third-order stations of the International Classification.
Auxiliary climatological stations : at which the observations are of the
same kind as at the normal climatological stations, but (a) less full, or
(b) taken once a day only, or (c) taken at other than the recognised
hours.
This classification serves to indicate the kind of differences which still
exist between meteorological stations at which observations are made of
elements besides rainfall, but the classification does not take effective
cognizance of types of stations which are now in operation for certain specific
purposes. For example :—
(a) At telegraphic reporting stations, observations are made daily at
some or all of the hours Oh., 3h., 6h., 9h., 12h., 15h., 18h., 21h. G.M.T.,
and telegraphed to the central office for use in the preparation of synoptic
charts for weather forecasting. Other kinds of observations have been
introduced, particularly cloud height, change in barometer in last 3 hours,
visibility, sea disturbance.
(b) At stations on airfields, observations are often made every hour
for the benefit of pilots. Visibility, cloud, and the motion of upper
currents are important items of the programme. In addition, these
stations are equipped as first-order stations and a corresponding pro­
gramme of work is carried out.
181
STANDARD ATMOSPHERIC CONDITIONS FOR DESIGN PURPOSES
(May, 1941)
Air Density at Zero Height (Barometer 1013-2 mb., Temp. 15° C.) is 0-07652 lb./ft.»
Sub-arctic
winter
I.C.A.N. condition s
Height
feet
Relative
Pressure pressure
Basis
°C.
Relative
density
°C.
Relative
density
Temperate
summer
Tropical
summer
0 C.
Relative
density
0 C. Relative
density
0-960
0-932
0-905
0-878
0-852
0-826
0-801
0-776
0-753
0-730
41-00 0-917
39-02 0-890
37-04 0-864
35-06 0-838
33-08 0-813
31-09 0-788
29-11 0-764
27-13 0-740
25-15 0-717
23-17 0-695
21-19 0-674
19-21 0-651
17-23 0-631
15-24 0-610
13-26 0-591
11-28 0-571
9-30 0-553
7-32 0-534
5-34 0-517
3-36 0-499
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
1-000
0-964
0-930
0-896
0-864
0-832
0-801
0-771
0-743
0-715
15-00
13-02
11-04
9-06
7-08
5-09
3-11
1-13
- 0-85
- 2-83
1-000
0-971
0-943
0-915
0-888
0-862
0-836
0-810
0-786
0-762
-20-0
-18-0
-17-0
-16-4
-16-2
-16-4
— 17-1
-18-3
-20-2
-22-1
1-138
1-089
1-046
1-006
0-969
0-934
0-901
0-872
0-846
0-821
10,000
11,000
12,000
13,000
14,000
15,000
16,000
17,000
18,000
19,000
0-688
0-661
0-636
0-611
0-587
0-564
0 • 542
0-520
0-499
0-479
- 4-81
- 6-79
- 8-77
-10-76
— 12-74
-14-72
-16-70
-18-68
-20-66
-22-64
0-739
0-715
0-693
0-671
0-650
0-629
0-609
0-589
0-570
0-551
-24-0
-26-0
-28-0
-30-0
— 31-9
-34-0
— 35-9
-37-7
-39-6
-41-2
0-796
0-771
0-748
0-724
0-701
0-680
0-658
0-636
0-616
0-595
7-19 0-707
5-21 0-684
3-23 0-663
1-24 0-642
- 0-74 0-621
- 2-72 0-601
- 4-70 0-582
- 6-68 0-562
- 8-66 0-544
— 10-64 0-526
20,000
21,000
22,000
23,000
24,000
25,000
26,000
27,000
28,000
29,000
0-459
0-441
0-422
0-404
0-387
0-371
0-355
0-340
0-325
0-311
-24-62
-26-61
— 28-59
— 30-57
-32-55
-34-53
-36-51
-38-49
— 40-47
-42-46
0-532
0-515
0-498
0-481
0-464
0-448
0-432
0-417
0-402
0-388
—43-1
-44-9
-46-5
—48-1
-49-7
— 51-3
-53-0
-54-6
-56-0
-57-5
0-575
0-557
0-537
0-518
0-499
0-482
0-465
0-448
0-431
0-416
-12-62 0-508
-14-61 0-492
-16-59 0-474
-18-57 0-458
-20-55 0-441
-22-53 0-427
-24-51 0-411
-26-49 0-397
-28-47 0-383
-30-46 0-369
1-38
- 0-61
- 2-59
- 4-57
— 6-55
- 8-53
-10-51
— 12-49
-14-47
— 16-46
0-482
0-466
0-450
0-434
0-419
0-404
0-389
0-375
0-362
0-349
30,000
31,000
32,000
33,000
34,000
35,000
36,000
37,000
38,000
39,000
0-297
0-283
0-271
0-258
0-247
0-235
0-224
0-214
0-204
0-194
-44-44
—46-42
— 48-40
-50-38
-52-36
— 54-34
-56-32
—56-50
— 56-50
-56-50
0-374
0-360
0-347
0-334
0-322
0-310
0-298
0-284
0-271
0-258
-59-0
—60-1
-61-4
-62-8
-64-0
-65-0
-65-0
-65-0
— 65-0
-65-0
0-400
0-383
0-369
0-353
0-340
0-326
0-310
0-296
0-282
0-269
-32-44
— 34-42
-36-40
-38-38
-40-0
-40-0
-40-0
—40-0
-40-0
-40-0
0-356
0-342
0-330
0-317
0-305
0-290
0-277
0-265
0-252
0-240
-18-44
-20-42
-22-40
-24-38
-26-36
-28-34
-30-32
-32-30
-34-28
-36-27
0-336
0-323
0-311
0-299
0-288
0-277
0-266
0-255
0-246
0-236
40,000
41,000
42,000
43,000
44,000
45,000
46,000
47,000
48,000
49,000
50,000
0-185
0-176
0-168
0-160
0-153
0-145
0-139
0-132
0-126
0-120
0-114
-56-50
-56-50
-56-50
-56-50
-56-50
— 56-50
-56-50
-56-50
-56-50
-56-50
-56-50
0-246
0-234
0-223
0-213
0-203
0-193
0-184
0-176
0-167
0-160
0-152
-65-0
-65-0
-65-0
-65-0
-65-0
-65-0
-65-0
-65-0
-65-0
-65-0
-65-0
0-256
0-244
0-232
0-222
0-211
0-201
0-192
0-183
0-174
0-167
0-158
-40-0
-40-0
— 40-0
-40-0
-40-0
—40-0
-40-0
-40-0
-40-0
— 40-0
-40-0
0-229
0-218
0-208
0-198
0-189
0-179
0-172
0-163
0-156
0-148
0-141
-38-25 0-227
-40-23 0-218
-42-21 0-209
-44-19 0-201
-46-17 0-194
-48-15 0-186
-50-13 0-179
-52-11 0-173
-54-09 0-165
-56-08 0-160
-58-06 0-153
27-00
25-02
23-04
21-06
19-08
17-09
15-11
13-13
11-15
9-17
STATISTICS
182
(c) At agricultural-meteorological stations (" crop-weather " stations)
special attention is given to the temperature of the soil at depths of
4 in. and 8 in.
(d) At health-resort stations, special observations are made in the late
afternoon mainly for purposes of publicity. These observations are
telegraphed to London and issued to the daily press as a special bulletin.
Statistics.—Accumulations of numerical facts. The science of statistics
is concerned with the development of methods of dealing with numerical
data (generally in large quantities), representing their essential features
by a small number of PARAMETERS, and determining the relations between
two or more variables. See STANDARD DEVIATION, NORMAL LAW (OF ERRORS),
CORRELATION.
Stefan's Law.—The law, discovered empirically by Stefan and afterwards
demonstrated theoretically by Boltzmann, that the total radiation in all
directions from an element of a perfect radiator is proportional to the fourth
power of its absolute temperature. See RADIATION, Black-body radiation.
Steppe.—A name given to the grassy, treeless plains in Russia and Siberia.
The word is sometimes extended to mean similar plains and regions of semiARID climate elsewhere.
Storm.—The term " storm " is commonly used for any violent atmos­
pheric commotion, such as a violent gale, a thunderstorm, a line-squall, a
rainstorm, duststorm or snowstorm.
Winds with velocity between 64 and 75 mi./hr. (force 11 on the
BEAUFORT SCALE) are classed as storm winds
Storm Cone.—See GALE WARNING.
Storm, Eye of.—See EYE OF STORM.
Stratocumnlus (Sc.).—A layer or patches composed of globular masses or
oils ; the smallest of the regularly arranged elements are fairly large; they
are soft and grey, with darker parts. These elements are arranged in groups,
in lines, or in waves, aligned in one or in two directions. Very often the rolls
are so close that their edges join together ; when they cover the whole sky—on the
continent, especially in winter—they have a wavy appearance.
Stratosphere.—The external layer of the atmosphere, in which there is
no convection. It has also been called the advective region for this reason.
The temperature of the air generally diminishes with increasing height until
a point is reached where the fall ceases abruptly. Above this point lies the
stratosphere, which is a region in which the temperature is practically constant
in the vertical direction. The term " isothermal layer" has sometimes
been applied to it, but this is misleading, as the temperature changes in a
horizontal direction. Further, a large proportion of ascents with sounding
balloons made in different parts of the world reveal an increase of temperature
with height in the lowest layers of the stratosphere. The horizontal tem­
perature gradient is never large, but an inspection of the diagram reproduced
under SOUNDING BALLOON, representing the result of a large number of
soundings in the British Isles, shows that the range of temperature in the
stratosphere equalled or exceeded the range on the earth's surface in that set
of observations.
We are still without any effective explanation of the origin of differences
of temperature which are found in the stratosphere ; they must probably
be classed among the most fundamental characteristics of the general circu­
lation of the atmosphere and among the primary causes of the changes of
weather, out hardly any light has been thrown on the mechanism of the
process.
Reliable observations of humidity in the stratosphere are few, but those
made by Brewer and others* over England using the Dobson-Brewer
* DOBSON, G. M. B., BREWER, A. w. and CWILONG, B. M. ; Meteorology of the lower
stratosphere. Proc. roy. Soc., London, A, 185, 1946, p. 144.
183
SUN PILLAR
frost-point hygrometer gave relative humidities of only a few per cent, and a
mixing ratio of the order of 0-01 gm./Kg. The observations were restricted
to occasions of relatively low TROPOPAUSE.
The height at which the stratosphere commences is often about 10 Km.,
but varies. It is higher in the regions nearer the equator. See TROPOSPHERE.
Stratus (St.).—A uniform layer of cloud, resembling fog, but not resting on
the ground. When this very low layer is broken up into irregular shreds it is
designated fractostratus (Fs.).
Subsidence.—The word used to denote the slow downward motion of
the air over a large area which accompanies DIVERGENCE in the horizontal
motion of the lower layers of the atmosphere. The greatest divergence is
from regions of rapidly rising barometer and the subsidence is probably of
the order of 100 to 200 ft. per hour in many cases. In stationary unchanging
anticyclones the subsidence is due to the outward air flow at the earth's
surface only, and is then very much slower. The subsiding air is warmed
dynamically (see ADIABATIC) and its relative humidity therefore becomes
low, occasionally falling below 10 per cent, at about 4,000 to 6,000 ft. after
prolonged subsidence. The downward movement and consequent warming
increases with height, up to 10,000 ft. or perhaps more, so that the LAPSErate of temperature is decreased, and INVERSIONS are often developed. The
vertical velocity is obviously zero at the ground, but turbulence often mixes
up the lower layers and brings some of the warm dry air to the ground.
Subsidence normally results in fine dry weather, but fog, stratus or stratocumulus clouds may occur in certain conditions.
Sumatra.—A squall which occurs in the Malacca Strait, blowing from
between SW. and NW. There is a sudden change of wind from a southerly
direction and a rise in speed is accompanied by a characteristic cloud
formation—a heavy bank or arch of cumulonimbus which rises to a great
height. These squalls usually occur at night, and are most frequent between
April and November. They are generally accompanied by thunder and
lightning and torrential rain. There is a sudden fall of temperature at the
moment the squall arrives.
Summer.—See SEASONS.
Sun.—A luminous sphere around which the earth moves in an orbit at an
average distance of 92,900,000 m^les. It is 865,000 miles in diameter, 332,800
times as massive as the earth and 1-41 times as dense as water. According
to astrophysical theory the sun has a temperature range from 46,000,000° C.
at the centre to 6,000° C. in the outer shell. The outer shell can be observed
from the earth, and its absorption spectrum indicates the presence of all
terrestrial elements although these could not exist as atoms at the centre.
There is also an outer corona which gives Fraunhofer spectrum lines. The
RADIATION emitted from the sun ranges from the far ultra-violet to the far
infra-red giving a spectral distribution of a black-body radiator at about
6,000° C. The surface of the sun is subject to disturbances known as sunspots.
Sun-dogs.—A popular name for mock suns or PARHELIA.
The origin of the name sun-dog is not known. Possibly it implies that
a parhelion follows the sun through the sky in the course of the day like a
dog its master. The drawback to this explanation is that as a general rule
sun-dogs do not last very long.
Sun Pillar.—A vertical column of light above (and sometimes below)
the sun, most often observed at sunrise or sunset. The colour may be white,
pale yellow, orange or pink. The phenomenon, which is due to the reflection
of sunlight from small snow crystals, may be seen over a wide area. On
March 13, 1924, it was observed all over the south of England from Cornwall
to Norfolk.
SUNRISE, SUNSET
184
Sunrise, Sunset.—The times at which the sun appears to rise and set,
in consequence of the rotation of the earth on its axis. Owing to the effect
of atmospheric refraction, which increases the apparent angular altitude of
the sun when near the horizon by about 34', sunrise is earlier and sunset
later than geometrical theory indicates. There is a further uncertainty
caused by the fact that the sun has an appreciable diameter so that time
elapses between the first and last contacts with the horizon. For meteorological
purposes allowance is made for refraction and it is assumed that sunrise
and sunset occur when the centre of the sun's disc is on the horizon.
The times of sunrise and sunset vary with the latitude and with the
declination of the sun. Diagrams illustrating the variations, so far as the
British Isles are concerned, are given in the " Meteorological Observer's
Handbook ".
Sunrise and Sunset Colours.—The general explanation of the variety of
colours that are to be seen in the sky about the time of sunrise or sunset
is as follows. White light such as that from the sun may be regarded as
composite, the constituents being light of all the colours of the spectrum.
When the light waves meet obstacles in their course, such obstacles as the
molecules of the atmospheric gases or larger obstacles such as particles of
dust, the waves are broken and secondary waves proceed in all directions
from the obstacles. The direct light is therefore reduced in strength and the
further the light goes through an atmosphere of such obstacles the more the
strength is reduced, the energy being used up in producing scattered light.
This effect is more pronounced with blue light, for which the wave-length
is short, than with red light foi which the wave-length is longei. Accordingly
a beam of white light passing through air loses the constituents of shorter
wave-length and becomes yellow, then orange, and finally red.
This accounts for the changing colour of the sun as it nears the horizon.
The scattering by air alone merely makes the setting sun yellow but if there
is dust in the air or even the nuclei on which water vapour is condensed
then the sun becomes orange or red before it sets.
Clouds which are illuminated by the light from the setting sun are also
red, whilst other clouds which are illuminated by scattered light in which
the blue constituents are present are white or grey ; higher clouds illuminated
by light which has only passed through less dense and cleaner air may also
appear white.
The colours of the sky itself are to be explained in the same way. When
sunlight has already travelled a great distance through the lower atmosphere
it has lost the constituents of short wave-length, and in the light which remains
to be scattered the longer wave-lengths predominate. Further in the passage
of scattered light to the eye of the observer the longer wave-lengths have
the preference.
When we look at the sky in a particular direction we receive light which
has been scattered by the atmosphere at all heights ; in the light scattered
at great heights blue may predominate, whilst that coming from the lower
levels may be red. The combination of light from both ends of the spectrum
gives us purple. On the other hand in other parts of the sky the middle
wave-lengths may predominate and the resulting colours are green or yellow.
It may even happen that the result is practically white as in the Ddmmerungschein sometimes observed above the sun when it is near the horizon.
Detailed descriptions of the changing illumination of the sky about the
time of twilight have been published by several observers, among the most
detailed being that in a work by Gruner and Kleinert (Hamburg, 1927)
devoted to observations in Switzerland.
The popular and ancient belief that a " red sky at night " is a good
prognostic of fine weather is probably justified since the " red sky " implies
present fine weather over a considerable area to the west. Statistical investi­
gations in London bear this out to some extent, and also the tendency for
a " red sky in the morning " to be followed by rain. (See London, Met.
Mag., 61, 1926, p. 16.)
185
SUNSPOT NUMBERS
Sunshine.—RADIATION from the sun is a mixture of radiations of different
wave-lengths at different intensities. The sunshine which renders objects
visible to the eye comprises that portion of the whole of the radiation from
the sun which has wave-lengths varying from about 0-4[i (\JL = one thousandth
of a millimetre) to about 0-8[z, the former corresponding with violet light
and the latter with red light, while in between the two are found the other
" colours of the rainbow " in the order indigo, blue, green, yellow, orange.
As received at the confines of the earth's atmosphere the intensity of the
violet light is much greater than that of the red light, but the radiation of
shorter wave-length is more readily scattered by the earth's atmosphere than
that of longer length, hence the proportions at the surface are very different.
When the sunlight has to pass through a long length of dust-laden atmosphere,
as at sunset in many cases, the longer wave-lengths so preponderate that red
is the dominant colour of the setting sun.
The therapeutic qualities of ultra-violet light have received much
attention of late. Sunshine rich in this light is found at high levels above
the lower and denser layers of the atmosphere.
Records obtained in the British Isles with the Campbell-Stokes recorder
show that sunshine is more abundant on the coast than inland, and that,
broadly speaking, the mean sunshine per day increases from the north towards
the south in all months of the year, in spite of the fact that at mid-summer
the possible daily duration of sunshine in the north of Scotland is about
two hours greater than in the south of England.
Low records are also obtained in industrial areas, due to obscuration by
smoke, and in the mountainous areas of western Scotland, the Pennines and
the west of Ireland. In these areas the mean daily duration in the period
1906-35 was less than 3-5 hours per day (29 per cent, of the possible duration).
On the south coast of England the mean daily duration is locally as high as
5-0 hours (41 per cent, of the possible). In the sunniest areas, six hours or
more of sunshine are recorded on about one day in three.
Monthly and annual averages for numerous stations are given in Averages
of Bright Sunshine for the British Isles published quinquennially. Charts and
discussions of the data will be found in " The distribution over the British
Isles of the average duration of bright sunshine" by J. Glasspoole and D. S.
Hancock (London, Quart. J. R. met. Soc., 62, 1936, p. 247), " The frequency
of days with specified duration of sunshine " by E. G. Bilham and Miss L. F.
Lewis (London, Prof. Notes, No. 69, 1935), and " The climate of the British
Isles " by E. G. Bilham (Macmillan, 1938).
Sunshine Recorder.—An instrument for recording the duration of bright
sunshine. In the Campbell-Stokes recorder a spherical glass lens focuses
the solar image on a graduated card held in a frame of special design. The
duration of sunshine is indicated by the length of the burnt track of the
image. (See " Meteorological Observer's Handbook.")
Sunspot Numbers.—The numbers which are used to represent the varia­
tion in the sun's surface from year to year as regards spots. The occurrences
of dark spots, sometimes large, sometimes small, which are to be seen from
time to time on the sun's face between its equator and forty degrees of latitude
north or south, have long been the subject of observation. An irregular
periodicity in their number was discovered by Schwabe of Dessau in 1851,
using 25 years of observation. Professor R. Wolf of Zurich, by means of
records in a variety of places, made out a continuous history of the sun's
surface from 1610 to his own time, which is now continued by his successor,
Professor Wolfer. The sunspot number N is obtained by the formula
N = k (10^ + /), in which g is the number of groups of spots and single
spots, / is the total number of spots which can be counted in these
groups and single spots combined, k is a multiplier representing " personal
equation " which depends on the conditions of observation and the telescope
employed. For himself, when observing with a three-inch telescope and a
power of 64, Wolf took k as unity.
SUPERCOOLING
186
The method of obtaining the number seems very arbitrary, but from the
examination of photographic records by Balfour Stewart and others, it is
proved that the numbers correspond approximately with the " spotted
area " of the sun. One hundred as a sunspot number corresponds with
about 1/500 of the sun's visible disc covered by spots, including both umbras
and penumbras (see " The Sun," by C. G. Abbot, 2nd edition 1929).
Spots are now regarded as vortical disturbances of the sun's atmosphere.
The sense in which the vortices rotate can be determined by spectroscopic
observations. It is doubtful whether individual sunspots have any direct
influence on terrestrial phenomena, but the epoch at which sunspots are
numerous are those at which other forms of solar activity develop. The
clearest evidence for this association is in the case of auroral displays and
magnetic storms. The amplitude of the regular diurnal changes in terrestrial
magnetism, even on quiet days free from magnetic storms, is increased at
the epochs of high sunspot numbers. There is evidence that the amount of
heat received from the sun is higher at such epochs, and that in certain
tropical regions rainfall and cloudiness increase.
The mean interval between the maxima in sunspot numbers is 11-1 years
but the recurrence is by no means strictly periodic ; the maxima are not of
equal strength and the interval fluctuates between 9 and 13 years.
The following is the list of sunspot numbers since 1750 :—
TABLE OF SUNSPOT NUMBERS, 1750-1944*
0
1
2
3
4
5
6
7
8
9
1750
1760
1770
1780
1790
83
63
101
85
90
48
86
82
68
67
48
61
66
38
60
31
45
35
23
47
12
36
31
10
41
10
21
7
24
21
10
11
20
83
16
32
38
92
132
6
48
70
154
131
4
54
106
126
118
7
1800
1810
1820
1830
1840
14
0
16
71
63
34
1
7
48
37
45
5
4
28
24
43
12
2
8
11
48
14
8
13
15
42
35
17
57
40
28
46
36
122
62
10
41
50
138
98
8
30
64
103
124
2
24
67
86
96
1850
1860
1870
1880
1890
66
96
139
32
7
64
77
111
54
36
54
59
102
60
73
39
44
66
64
85
21
47
45
64
78
7
30
17
52
64
4
16
11
25
42
23
7
12
13
26
55
37
3
7
27
94
74
6
6
12
1900
1910
1920
1930
1940
10
19
38
36
68
3
6
26
21
48
5
4
14
11
31
24
1
6
6
16
42
10
17
9
10
64
47
44
36
54
57
64
80
62
104
69
114
48
81
78
110
44
64
65
89
* See BRUNNER W.; Tabellen und Kurven zur Darstellung der Haufigkeit der Sonnenflicken in den Jahren 1749-1944, Astr. Mitt., Zurich, No. 145, 1945, Table 1.
Supercooling.—Although, in ordinary circumstances, water changes to ice
when its temperature foils below the " freezing point," 0° C., it is possible,
by careful control, to preserve the liquid state at much lower temperatures—
the water is then said to be supercooled.
Supercooling is for most purposes little more than a scientific curiosity,
but the small droplets within a cloud provide an important exception ; it
is, indeed, a normal condition within a cloud at temperatures well below
the ordinary freezing point. Altocumulus clouds are for the most part water
clouds although their average temperature approaches zero Fahrenheit. All
clouds which penetrate vertically through the 32° F. isothermal surface
(average height, 6,000 ft. in England), contain supercooled droplets, and it is
necessary to go to cirrus levels (average temperature about — 10° F.) to find
clouds almost invariably in the form of ice crystals.
187
SURFACE OF SUBSIDENCE
Various theories have been propounded to explain the supercooling by
introducing such factors as surface tension, salt solution and electrification.
While these are known to depress the normal freezing point they fail, however,
to provide an effect of the necessary magnitude. There is, furthermore, good
reason to believe that such theories are unnecessary, except to indicate con­
tributory factors. As long ago as 1724, Fahrenheit succeeded in supercooling
a glass bulb full of water; more recently, water in bulk has been cooled to
— 6° C. (21° F.) and small droplets to - 20° C. (- 4° F.). Liability to super­
cooling must be regarded, then, as a natural physical property of water. The
freezing process requires that the molecules shall set themselves in the
characteristic orderly manner defining the crystalline structure, and provided
the liquid is practically undisturbed, there is no mechanism for initiating the
process. The supercooled state is said to be unstable, but more strictly it
should be regarded as just stable—a very small but nevertheless finite
disturbance appears to be necessary to overcome this stability. A small cloud
particle which virtually floats in the air is subject to very small internal
disturbances and remains stable as a liquid. As would be expected the larger
the drop the less likely it is to remain undisturbed and large raindrops rarely
undergo much supercooling without freezing.
Amongst the many minor properties of water which have a profound
bearing on meteorology, its liability to supercooling takes an important
place. Hail, rime and glazed frost, including the dangerous phenomena of
ice accretion on aircraft in flight, are all dependent upon the occurrence of
supercooling. There are grounds for believing that the formation of raindrops
generally requires snowflakes falling and melting, and that the flakes are
formed in a region of small supercooled droplets, and grow as the result of
the difference of vapour pressure over water and ice. On reaching a region
of supercooled droplets, these flakes grow rapidly owing to the difference
in vapour pressure over water and ice. Thus the possibility of supercooling
has probably an important effect on precipitation and, therefore, on
many fundamental problems of the atmosphere.
Supersaturation.—See CONDENSATION.
Surface of Discontinuity.—In idealised theory, this is regarded as a real
impenetrable boundary surface separating the atmosphere into two portions
which differ in temperature, humidity and wind velocity. Actually the
discontinuity is usually primarily one of temperature and therefore of density,
the pressure at any point on the surface being the same on the two sides.
If the surface is sloping, the isobaric surfaces are refracted at the surface of
discontinuity, so that there is a discontinuity of wind velocity. The term
is most often used in connection with FRONTS. The frontal surfaces are
frequently not very sharp, but the term discontinuity is justified when one
is dealing with large-scale phenomena. The SURFACES OF SUBSIDENCE are
generally sharp.
Surface of Subsidence.—A SURFACE OF DISCONTINUITY is often developed
in a subsiding mass of air, which is usually polar in origin. The primary
cause is the dynamical warming due to the descending motion, as the result
of which the air above the discontinuity has a low relative humidity.
Frequently there is a sharp temperature INVERSION. These discontinuities
are most frequent in anticyclones, and are then either horizontal or inclined
at a very small angle. They also frequently occur some distance below a
frontal surface (see FRONT), sloping down towards the line of the front on the
ground, but inclined at only a small angle, usually of the order of 1 in 200.
This angle is quite sufficient to give a pronounced wind discontinuity. The
term " surface of subsidence " was introduced to apply more especially to
such sloping surfaces, and to distinguish them from the frontal surfaces
proper. The origin of the sharp inversion is not yet clearly understood,
but it is frequently connected with the cloud-sheet (either continuous or broken)
SURGE
188
which is found under the inversion, at least while it is developing. These
clouds are maintained by convection and turbulence, which are especially
active when POLAR AIR is passing over a warmer sea surface, extending up
to a limit which becomes lower and sharper as the air subsides. Above this
level the descending air warms at the dry ADIABATIC rate, and below it at a
lower average rate, depending on the amount of cloud. Radiation also helps
to sharpen the inversion.
Surge.—First used by Abercromby to denote a general change of pressure
superposed upon the changes which are due to the movements of DEPRESSIONS
and ANTICYCLONES. Sometimes between two consecutive weather maps
the isobaric systems are seen to have travelled across the map but to have
changed their shape little, while on closer examination, it is found that pressure
over the whole area has risen. A change of this type is referred to as a " surge."
Swell.—Swell is wave motion in the ocean caused by a disturbance which
may be at some distance away; the swell may persist after the originating
cause of the wave motion has ceased or passed away. It often so continues
for a considerable time with unchanged direction, as long as the waves travel
in deep water. The height of the waves rapidly diminishes but the length
and velocity remain the same, so that the long low regular undulations,
characteristic of swell, are formed. Swell is often observed to have a wave­
length greatly in excess of that of waves seen during a storm : the probable
explanation is that the longer waves are then masked by the shorter and
steeper storm waves. Swell observations are useful as denoting the direction
in which sea disturbance due to tropical cyclones or other storms has taken
place.
Symbols, International Meteorological—See WEATHER.
Sympiesometer.—A form of BAROMETER, now obsolete, in which the
greater part of the atmospheric pressure was balanced by enclosing air
above the liquid. By using glycerine as the barometric fluid, a very open
scale was obtained in a compact instrument, but temperature effects were
troublesome.
Synoptic.—An adjective derived from the noun Synopsis " a brief or
condensed statement presenting a combined or general view of something."
In meteorology the word is generally used either in connexion with charts
or with wireless or telegraphic messages. A synoptic chart shows the weather
conditions over a large area at a given instant of time. Such charts are
discussed under WEATHER MAP. A synoptic message contains reports from a
selection of observing stations in a large area chosen to give a represen­
tation of the weather over that area.
Temperature.—The condition which determines the flow of heat from
one substance to another. Temperature must be clearly distinguished from
HEAT, heat being a form of energy, while temperature is a factor which affects
the availability of the energy. Temperature may be measured according to
various scales, those in most frequent use being FAHRENHEIT, CENTIGRADE
and ABSOLUTE TEMPERATURE. The word is used in a special sense in
ACCUMULATED TEMPERATURE.
For the highest and lowest recorded air temperatures see EXTREMES.
Temperature Gradient.—When observations were first taken in the free
air and the change of temperature with height became known, the term
" temperature gradient " was used to express this change. The term thus
meant the change of temperature in unit height. The word GRADIENT is
used in other cases to denote the change of elements in the horizontal. Thus
by " pressure gradient " is meant change of pressure per unit horizontal
distance. It is better that the meaning of the word should be confined to
change in the horizontal and the term " lapse-rate " has therefore been
introduced for changes of temperature in the vertical leaving the term
" temperature gradient " for changes of temperature in the horizontal.
Tendency.—See BAROMETRIC TENDENCY.
CO
tn
so
g
FIG. 27.—Swell proceeding ahead of a tropical storm (actually approaching Nassau, Bahamas from West Indian hurricane. Note that
the swell waves are long, low and long-crested. The wind is blowing almost at right angles to the direction of motion of the swell (i.e.
paiallel to the ciests), as is made clear by the breaking of sea waves in the foreground.
Reproduced bv the courtesy of the U.S. Nary
30
00
189
TERRESTRIAL MAGNETISM
Tension of Vapour.—An old term for the pressure exerted by a vapour.
See AQUEOUS VAPOUR.
Tenuity Factor.—In BALLISTICS, the ratio of the density of air having the
observed pressure at the surface, and temperature equal to the ballistic
temperature, to the density of air at pressure 30 in. of mercury and tempera­
ture 60° F.
Tephigram.—A diagram on which is represented the condition of the
atmosphere at different levels in terms of its temperature t and entropy <|),
hence the name t (j)gram.
Tercentesimal Scale.—The name adopted by Sir Napier Shaw for the
approximate absolute scale of temperature, the tercentesimal temperature
being obtained by adding 273° to the Centigrade temperature. See ABSOLUTE
TEMPERATURE.
Terrestrial Magnetism.—The earth has been happily described as a great
magnet ; the distribution of magnetic force at its surface may as a first
approximation be regarded as that due to a uniformly magnetized sphere,
whose magnetic axis, however, is inclined at some 10° or 12° to the axis of
rotation (polar axis) of the earth. The actual magnetic poles are the one to
the north of Canada (between Victoria and Baffin Islands), the other in the
Antarctic (South Victoria Land). At these poles the dip needle is vertical,
and the horizontal component of magnetic force vanishes ; the compass
needle has no guiding force and points anyhow. These poles do not coincide
with the poles of the axis of uniform magnetization, nor are they at opposite
ends of a diameter. At what may be called the magnetic equator, which is
nowhere very far (comparatively speaking) from the geographical equator,
the vertical force vanishes : the dip needle is horizontal, and the horizontal
force has its largest values. The horizontal force in London is only about
half that where the magnetic equator crosses India. The direction of the
horizontal component of magnetic force is exactly true north-south (i.e., the
magnetic declination is zero) only along two lines or narrow belts. One of
these crosses Canada, the United States, and South America ; whilst the
other after crossing Finland, eastern Russia and Egypt, encircles China, Japan
and a large part of Siberia in a large oval loop, and thence passes near to
Sumatra and across western Australia towards the south magnetic pole.
Within the region bordered on the west by the first and on the east by the
second of these agonic lines, and within the east-Asian loop, declination is
westerly. If we travel westward from the Asiatic-European agonic line the
declination becomes increasingly westerly until we pass the western limits
of Europe. Thus, the mean values for 1936 are at Helwan (Cairo)* 0° 13' E.,
at Seddin 4° 45'W., at Val-Joyeux (Paris) 9° 57' W., at Abinger (Surrey,
England) 11° 20' W., at Eskdalemuir (Dumfriesshire, Scotland) 13° 47'W.,
at Valentia (Kerry, Ireland)* 16° 22' W., and at Coimbra (Portugal)*
13° 3' W. The values of the magnetic elements and the positions of isomagnetic
lines change gradually with time, having what is known as " secular change "
(see SECULAR TREND), the rate and the sense of which vary with time and
locality. At London a change from easterly to westerly declination occurred
about 1760; westerly declination increased to 24-6° in about 1818, since
when there has been a continuous swing of the north end of the needle towards
east. The rate of decrease in westerly declination in the British Isles has
increased in recent years, the average annual rate since 1920 being 12' as
compared with about 9' between 1910 and 1920 and 5' between 1900 and
1910. In the same area the horizontal force has been decreasing since 1911,
but changes in inclination (dip) are small at present. Various theories (including
the existence of a permanently magnetized core, the rotation of electric
charge or of separate electric charges, direct magnetization by rotation) have
been advanced in explanation of the cause of the earth's magnetism but none
has been adequate to explain all the observed facts.
* Results derived from absolute observations only.
TERRESTRIAL MAGNETISM
190
Diurnal variation.—In addition to secular change the earth's magnetic
field experiences regular diurnal variation and also frequent irregular changes
of varying magnitude. In magnetically quiet conditions, and more especially
in non-polar localities, the regular diurnal changes are largest in daylight
hours. In the northern hemisphere the prominent part of the daily variation
in declination is a westerly movement from about 7h. to 13h. (local time),
followed by a slower easterly movement. Speaking generally, the range of
the diurnal variation in declination is least near the equator, where it may
average less than 3', and greatest near the magnetic poles, where it may
exceed 1°. On the average quiet day in England the lowest value of horizontal
force occurs between lOh. and noon, and the highest value in the evening or
early forenoon according to the season ; dip is greatest at from 9h. to llh.
(earlier in summer than in winter) and least at about 6h. in winter and at
18h. to 20h. in summer. Not only the type but the range of the diurnal
variation varies with the season : the range being least in winter and largest
in one of the summer months. At Kew Observatory, for instance, the diurnal
inequality ranges in December and in the summer of the average year of the
period 1890-1900 were respectively :—declination, 3-3', 11'; dip, 0-6', 2-1';
horizontal force, • 00011, • 00039 C.G.S. units. The range of the regular diurnal
variation and the mean absolute daily range vary considerably from year
to year, showing a remarkably similar progression to that of sunspot frequency.
In a year of sunspot maximum the range of the diurnal variation may be
50 per cent, or more larger than a few years earlier or later at sunspot
minimum. The relative increase in diurnal inequality range associated with
increase in sunspot number tends to be greatest in winter months ; it appears
to vary with locality and is not the same for all the magnetic elements. In
magnetically disturbed conditions diurnal variations of a characteristic type
also occur. These differ from the quiet-day variations in appearance and
dimensions to an extent depending upon the degree of disturbance ; in par­
ticular, the displacements occurring during the night-time hours become of
the same order as those during the daytime. There is also a small lunar
variation.
Disturbance.—Disturbance or irregular fluctuation is present in varying
degree on all but a comparatively few days, and is more prominent as the
magnetic polar regions are approached. Disturbance at Eskdalemuir is on
roughly twice the scale of that at Kew ; and disturbance at Lerwick again is
on quite twice the scale of that at Eskdalemuir. Irregular disturbance changes
in high latitudes are not only larger than, but may differ in sense and general
character from, those simultaneously in progress in lower latitudes. In the
latter regions of the earth disturbance changes tend to be largest in the
horizontal component of force, but irregular changes in vertical force become
increasingly important in higher latitudes. The frequency of occurrence of
disturbance is greater in equinoctial than in other months ; and, in Europe,
greater between 15h. and 4h. G.M.T. than at other hours, being least during
the three or four hours before noon. The incidence of disturbance is greater
in sunspot maximum than in sunspot minimum years, but the numerical
relationship with the available measures of solar activity is smaller in the
case of measures of magnetic activity than in the case of the range of the
diurnal inequality. Disturbed, and also quiet, magnetic conditions tend to
recur after the lapse of one or more solar rotation periods. No very close
relationship has been established as yet between the degree of magnetic
disturbance on a given day and solar conditions (as at present observed) on
the same or on the few preceding days.
Causes of magnetic variations.—According to a widely accepted theory,
due in the first instance to Balfour Stewart, the solar diurnal (and the much
smaller lunar diurnal) magnetic variations are ascribed primarily to the
existence of electric currents circulating in the upper atmosphere, such
currents being engendered by electromotive forces induced by the largescale tidal or quasi-tidal motions of the highly ionized upper atmospheric
191
THERMOGRAM
strata across the permanent magnetic field of the earth. The necessarily
high degree of ionization of the atmospheric shell in question is attributed
to the agency of solar ultra-violet radiation, and the known facts as to the
magnetic diurnal variations suggest that the degree of ionization and, there­
fore, the intensity of the ionizing agent, varies with season and with the
phase of the solar cycle. Magnetic disturbance reaches its maximum in the
polar regions and is apparently due to intense electric currents located in
the upper atmosphere near to the zones of maximum auroral frequency (see
AURORA). According to the Birkeland-Stormer theory the high ionization of
these polar belts is held to be produced by solar corpuscular emissions which,
owing to the deflective force exerted by the earth's magnetic field, impinge
on the atmosphere in a somewhat restricted region around each magnetic
pole, giving rise to aurora. It is now well known that there is a close associa­
tion between the problems of terrestrial magnetism and of radio-transmission
and it is probable that work in these allied fields will go far to elucidate
many of the mysteries as to the properties and also the movements of the
upper atmosphere. Certain of the inferences drawn from radio-transmission
and other data as to the properties of the highly ionized region of the atmos­
phere have led to the suggestion that the regular diurnal magnetic variations
and possibly those on disturbed days may be the result of the diamagnetic
effect caused by ions (produced by ultra-violet radiation) spiralling about the
earth's magnetic field in the region of long free paths. (See also London,
Quart. J.R. met. Soc., 59, 1933, pp. 3-15.)
Terrestrial Radiation.—See sub-heading under RADIATION.
Thaw.—The transition by melting from ice to water ; the term used to
indicate the break-up or cessation of a FROST. In the British Isles the final
disappearance of frost is due usually to the displacement of cold northerly
or easterly winds by warm winds from the Atlantic, e.g., the final break-up
early in March of the prolonged frost of February, 1895, was due to the
interruption of the air supply from the cold continent by the invasion as a
southerly wind of a relatively warm current of air associated with the approach
of a depression from the Atlantic. In more northern latitudes the " spring
thaw " is a periodic event, denoting the seasonal progression, the unlocking
of ice-bound seas, and the melting of the snow. In these latitudes in western
Europe, though not in Canada or Asia, the sun is generally at sufficient
altitude about noon, except near mid-winter, to effect a partial thaw by day,
even in the midst of a protracted frost, if the sky is clear.
Theodolite, Pilot Balloon.—An instrument consisting of a telescope
mounted to permit of rotation in altitude and azimuth, and fitted with
divided circles to permit of those co-ordinates being read. The telescope
is fitted with a right-angled prism so that the observer continues to look
horizontally into the eyepiece no matter what the altitude of the balloon.
Thermal Equator.—The line passing through the centre of the belt of
high temperature which exists near the geographical equator. The thermal
equator varies with the season, migrating north in the northern-hemisphere
summer and south in the sou them-hemisphere summer ; the amount of its
migration, however, is less than that of the sun. It is also considerably in­
fluenced by the distribution of land and sea and the effect of ocean currents ;
for this reason its mean position does not coincide with the geographical
equator, but lies to the north of it.
Thermal Wind.—The thermal wind in any layer of the atmosphere equals
the vector difference between the geostrophic wind at the bottom of the layer
and the geostrophic wind at the top of the layer.
Thermodynamics.—That part of the science of heat which deals with the
transformation of heat into other forms of energy and vice versa. See
ENERGY and ENTROPY.
Thermogram.—The continuous record of temperature yielded by a
THERMOGRAPH.
THERMOGRAPH
192
Thermograph.—A recording THERMOMETER. Many patterns are in use,
the sensitive member being either a bimetallic spiral (Meteorological Office
pattern), a Bourdon tube (Richard), a resistance element (Cambridge and
similar recorders), a steel bulb filled with mercury (Negretti and Zambra
distance thermograph) or an air bubble in the tube of a mercury-in-glass
thermometer (photographic thermographs).
Thermometer.—An instrument for determining temperature, usually by
means of the changes in volume of mercury or spirit contained in a glass
tube with a bulb at one end. Mercury is preferred in all cases when the
instrument is not required to indicate below — 40° F. or to show the
minimum temperature over an interval.
In meteorology, the temperature of the air is usually required and
precautions must be taken to avoid errors due to the effects of radiation,
etc. Sufficient accuracy for practical purposes is obtained by exposing
thermometers in a STEVENSON SCREEN. Alternatively, the thermometers
may be whirled or aspirated as in the Assmann PSYCHROMETER.
A spirit thermometer may be made to indicate the minimum temperature
by inserting in the stem, before sealing, a dumbbell-shaped light glass index,
and mounting the thermometer horizontally. As the temperature falls, the
index is drawn towards the bulb, but is left behind when the temperature
rises. A mercurial thermometer may be made to indicate the maximum
temperature by constricting the stem just above the bulb. Mercury is forced
through the constriction when the temperature rises, but is unable to return
when the temperature falls.
For special purposes, use may be made of changes in electrical con­
ductivity, of thermo-electric currents or of the dissimilar expansion of different
metals, for indicating or recording temperature.
Thickness lines.—Synoptic lines of constant height interval between
standard pressure surfaces. They are used in the process of graphical addition
for finding the contour lines of isobaric surfaces. Since on a hydrostatic basis
the mean temperature of the air column between two given pressure surfaces
is proportional to the height interval between them, the chart of thickness
lines gives a graphical representation of the distribution of mean temperature
between the specified pressures.
Thunder.—The noise which follows a flash of lightning, attributed to the
vibrations set up by the sudden heating and expansion of the air along the
path of the lightning, followed by rapid cooling and contraction. The distance
of the lightning flash may be roughly estimated from the interval that
elapses between seeing the flash and hearing the thunder, counting a mile
for every five seconds.
The long continuance of the thunder is explained by the fact that the
sounds from different parts of the lightning flash have different distances
to travel to reach the observer. The changes in intensity are partly due to
the crookedness of the flash but they may also be caused by variations in
the amount of energy developed along its course. The fact that what appears
to the eye to be a single flash may really be half a dozen flashes, all occurring
within a second or less and following the same path, complicates the question.
The sound may echo back from mountain sides but whether echoes from the
clouds add to the reverberation is doubtful.
The distance at which thunder can be heard is, as a rule, surprisingly
small, being usually less than 10 miles, though distances ranging up to
40 miles and over have been reported in extreme cases. Mr. R. S. Breton
writing to the Meteorological Magazine* from Tung Song, S. Siam, stated
that he had timed thunder reaching him 200 seconds after flashes from a
distant storm and that it was not rare for thunder to be heard there
180 seconds after the lightning. C. Veenemaf quotes observations of 255
and 310 seconds made at Norderney, Prussia, on September 5, 1899.
Observations made with the aid of balloons or on mountains have shown
that sound does not travel easily downwards, and for this reason thunder
* London, Met. Mag., 63, 1928, p. 113.
t Wetter, Berlin, 34, 1917, p. 192.
193
TIDE, INFLUENCE ON WINDS
caused by lightning flashes to the ground carries farther than thunder
caused by flashes in the clouds. The sound travels farthest at night when
the surface layers are in a stable condition, which is favourable for sound
transmission.
Thundercloud.—A towering cumulus cloud which is always associated
with thunderstorms. The technical name is CUMULONIMBUS. See also
CLOUDS.
Thunderstorm.—Thunderstorms are caused by the powerful rising currents
within fully developed CUMULONIMBUS clouds. In the British Isles they
are neither so numerous nor so severe as in the tropics. They are most
frequent in our eastern and midland districts, and a large majority, including
all the really violent examples, occur in the summer half-year, especially
from May to August. They are most frequent in the afternoons and evenings
of sunny days, and in such conditions the heavy rain or hail causes marked
local cooling and sometimes violent out-rushing squalls.
On the western coastal districts of the British Isles the storms are of a
somewhat different type and occur with nearly equal frequency in the summer
and winter seasons (see table on p. 194).
The conditions required for a thunderstorm are, first, an adequate supply
of moisture for cloud development (in the British Isles the cloud base is probably
always below 10,000 ft., and almost always below 6,000 ft.) ; secondly, a
LAPSE-rate of temperature in excess of the saturated ADIABATIC through a
range of height of not less than 10,000 ft. above the cloud base, and usually
much more in the case of severe storms. There are many hot summer days,
especially in anticyclones, when these conditions are not fulfilled, so that
the hottest summers are not in general the most thundery. The requisite
conditions exist most frequently in shallow DEPRESSIONS or COLS. Given
the favourable conditions up above, it is quite possible for severe storms to
develop at night, provided there is some initial impulse to start off the cloud
formation. This impulse is probably supplied by CONVERGENCE—an important
factor at any time of day. Sometimes the convergence occurs along COLD
FRONTS, which are often accompanied by thunderstorms, but only when the
upper air conditions are suitable. They do not therefore, constitute an
independent class of thunderstorms. The cold-front storms often sweep
broadside over the country, while some other storms develop along a belt
and move end on, occasionally lasting for some hours at one place. In general
thunderstorms drift with the wind at their level, but the variation of wind
with height, and the development and dissolution of parts of the cloud mass,
often make their movements complex. They occasionally travel at a high
speed (up to 50 mi./hr.) from some southerly point, while near the ground the
wind is relatively light, and usually blows from an easterly point. Many
travelling storms can be traced back to heated areas on the continent.
Showers in deep POLAR CURRENTS are often accompanied by thunder,
especially near the western seaboard in winter, and in eastern districts on
summer days, with a temperature frequently below normal even at the surface,
but with a much greater deficiency above. Such storms are usually of slight
to moderate intensity.
For an account of the distribution of thunderstorm frequency in other
parts of the world see London, Geophys. Mem,, No. 24, 1925.
Tide, Influence on Winds.—In some bays and estuaries where the rise and
fall of the tide is considerable a slight motion of the air, landward where
the tide is coming in or seaward where the tide is going out, is said to have
been observed, giving rise to an appreciable wind in calm weather or to an
increase in force and gustiness where the tide assists the direction of the
general wind at the time. The subject cannot, however, be said to have
really been investigated and such winds if they exist will be extremely local
and transitory in character. To establish them fully they would have to be
distinguished not only from the general wind prevailing over a wide area at
the time, but also from other local winds, in particular from land and sea
breezes. Sailors and others have also asserted that weather changes set in
with " the turn of the tide " and the upcoming tide in estuaries is said to
produce showers of a very local type, not extending far from the river banks,
(90565)
G
SCILLY
AND
IRELAND
SCOTLAND
CHANNEL
ISLANDS
O
C/3
H
«
H
«
0
X
w
%
H
to
•«!
W
NORTH
178
34
82
37
44
74
30
19
16
24
24
32
22
39
20
34
62
24
24
38
16
35
52
37
44
17
804
152
200
133
114
459
188
100
63
123
37
43
59
81
53
105
133
53
88
97
63
86
66
74
91
35
Sumburgh Hd. (Shetland) .
Deerness (Orkneys) . .
Stornoway (Lewis) . .
Wick (E. Coast)
Fort Augustus (Inland)
Dunrobin Castle (N.E.Coast)
Nairn (N.E. Coast)
Aberdeen (E. Coast)
Dundee (E. Coast)
Leith (E. Coast)
Laudale (W. Coast)
Rothesay (Island)
Glasgow (W. Coast)
Pinmore (Sub Coastal)
Douglas (Isle of Man)
Malin Hd. (N. Coast)
Blacksod Point (W. Coast) . .
Markree Castle (Inland)
Armagh (Inland)
Donaghadee (E. Coast)
Dublin (E. Coast)
Birr Castle (Inland)
Valentia (S.W. Coast)
Roches Point (S. Coast)
St. Mary's (Scilly)
St. Aubins (Jersey)
68
27
85
211
70
109
127
176
105
415
167
40
71
144
86
43
397
176
109
186
317
61
132
132
302
530
68
52
224
350
47
185
139
82
46
411
350
29
78
112
89
174
1,578
630
1,051
350
350
54
82
242
192
1,578
Summer Autumn Winter
Spring
Station
ENGLAND
Yarmouth (E. Coast)
Cambridge (Inland)
N. Shields (E. Coast)
Durham (Inland)
..
York (Inland)
Spurn Head (E. Coast)
Portland Bill (S. Coast)
in Pembroke (W. Coast)
SJ Falmouth (S.W. Coast)
%*
£*
B&
..
63
123
108
21
43
63
63
48
41
42
36
Margate (E. Coast)
St. Leonards (S. Coast)
Dover (S.E. Coast)
Southampton (Estuarine)
44
23
80
26
28
29
31
32
32
Stonyhurst (Inland)
Liverpool (W. Coast)
«w Llandudno (W. Coast)
Holyhead (W. Coast)
iS
<g«
« m
P
£8
,.
Spring
Loughborough
gS
<5 Cheltenham
3 z Oxford
Q
«
H
<n
NORTH­ EAST
Station
32
72
43
8
19
29
27
17
14
15
17
10
14
12
12
9
20
10
12
10
77
93
61
26
68
61
57
103
39
52
55
64
78
63
75
53
105
83
79
65
209
315
89
84
174
157
209
698
157
262
149
788
209
525
262
350
350
525
630
394
Summer Autumn Winter
Table of " odds against one," expressing the random chance of a thunderstorm on any day in the several seasons of the year.
Immunity from Thunderstorms in various parts of the United Kingdom as shown by observations extending mainly
over the 35 years 1881-1915.
H
g
W
»
a
195
TIME
on days when the weather conditions are such that precipitation is imminent.
Abercromby observed a well-defined tidal variation of the trade wind in Fiji
and Hazen found a marked increase of thunderstorms in the United States
with a rising as opposed to a falling tide. There is always the risk that
meteorological events may be related in the popular mind with tidal changes,
simply because the latter afford a convenient subdivision of the 24-hour
day with which to associate changes that may really have been distributed
over periods of several hours.
Tides, Atmospheric.—The attraction of the sun and moon should
theoretically raise tides in the atmosphere as well as in the hydrosphere. The
solar atmospheric tides are masked by the much greater thermal effects
but the lunar atmospheric tide exists as a very small independent wave,
which has been exhaustively investigated by Prof. S. Chapman (see e.g.
London, Proc. roy. Soc., A., 151, 1935, pp. 105-17). There are two maxima
and two minima in each lunar day ; the amplitude is greatest in the tropics
but nowhere reaches 100 microbars (0-1 mb.).
Time.—When the centre of the sun is due south of an observer, the time
is called 12h., or noon, local apparent time (L.A.T.).
The sun is said to
" transit " at this time. The interval between two consecutive transits
of the sun can be divided into 24 equal parts, and the times where the lines
of division fall are numbered 13h., 14h. . . . 23h., 24h. (or midnight),
lh., 2h. . . . 12h. Local apparent time is recorded by sunshine recorders
and sundials.
The interval of time between successive transits of the sun is not quite
constant, but goes through a cycle of changes during the year. This is due
to the two facts that the orbit of the earth is elliptical, and that the earth's
axis is not at right angles to its orbit. For details, books on astronomy
may be consulted. As it would be very inconvenient in daily life if the
length of the day varied in this way, astronomers have invented an imaginary
body called the " mean sun." The apparent position of this body is always
very close to that of the real sun, and on the average of a year the apparent
positions of the two coincide. The apparent motion of the mean sun is such
that it rotates uniformly round the earth at all times of the year. At a transit
of the mean sun the time is called 12h: (or noon) local mean time (L.M.T.).
The interval between successive transits is called a DAY, and each day is
divided into 24 equal parts called hours, which are numbered as before. All
hours are equal to one another, and all days are equal to one another in
duration. To get local mean time from local apparent time it is necessary to
add (or subtract if sign is minus) the so-called EDUATION OF TIME which is given
very accurately in the Nautical Almanac and elsewhere, and sufficiently
accurately for meteorological purposes in the " Meteorological Observer's
Handbook." The equation of time varies from about + 14J minutes in the
middle of February to about — 16\ minutes at the beginning of November.
Local mean time at Greenwich is called Greenwich Mean Time (G.M.T.).
The difference between the local mean times at two places depends only
on the difference in longitude between the two places. Since the earth makes
one complete rotation relative to the mean sun in 24 hours, 360° of longitude
corresponds with 24 hours, and therefore, 1° of longitude corresponds with
4 minutes of time. Hence L.M.T. for any place is derived from G.M.T. by
subtracting or adding a correction to G.M.T. at the rate of 4 minutes of time
for each degree of longitude of the place, according as the place is to the west
or east of the meridian of Greenwich.
In order to avoid the great inconvenience which would be produced if
every place adopted its own local mean time, it has been enacted that
Greenwich mean time should be used for all common purposes in this country,
except in summer, when clocks are advanced by one hour. See BRITISH
SUMMER TIME.
(90565)
G 2
TORNADO
196
The statement commonly made that the sun is in the south at noon is
thus seen to be true only when " noon " means 12h. local apparent time,
Considerable divergencies occur from place to place and from time to time
if " noon " means 12 o'clock by legal time.
In meteorological work Greenwich mean time and local mean time are
used as far as possible for purposes of defining the hours of observation—
summer time is not generally used. Meteorologists are closely interested
in good timekeeping, because punctuality is of importance in taking
meteorological observations. The records of self-recording instruments are
for many purposes useless unless marks are made on the records at known
times, so that allowance can be made for the irregularities in running of the
clock. Regulation of clocks in this country is effected by telegraphic, radiotelegraphic, and radiotelephonic signals originating from Greenwich
Observatory, where the standard clock is controlled by frequent observations
of the fixed stars. Time signals of high accuracy are also obtainable by
telephone.
Tornado.—(1) In west Africa, the squall which accompanies a thunder­
storm ; it blows outwards from the front of the storm at about the time
the rain commences. West African tornadoes occur most frequently during
the transition months between rainy and dry seasons.*
(2) A very violent counter-clockwise whirl of small area averaging a few
hundred feet in diameter, which gives wind velocities estimated to exceed
200 mi./hr. in some examples. Such tornadoes are most frequent in the
United States, east of the Rocky Mountains, especially in the central
plains of the Mississippi region ; they occur also in Australia, and are not
unknown in Europe, although less intense. In the United States tornadoes
are associated with thundery conditions, especially in SECONDARY
DEPRESSIONS, where the inter-action of winds with a large difference in
temperature gives opportunity for violent vertical convection. The tornado
is a local phenomenon, and usually travels towards the north-east with an
average speed of about 30 mi./hr. In the comparatively narrow track of the
tornado, trees are uprooted and buildings are completely destroyed, the large
and rapid decrease of pressure near the centre of the tornado allowing the air
inside buildings to produce almost an explosive effect. Heavy rain, and
generally thunder and lightning occur with the tornado.f
The few tornadoes known to have occurred in Europe have been of
comparatively small intensity, although they have caused a certain amount
of damage. Examples occurred in South Wales on October 27, 1913, in
Holland on August 10, 1925, and in London on October 26, 1928.
Torricelli, Evangelista.—The inventor of the barometer, born at Piancaldoli
in 1608. In 1641 he became amanuensis to Galileo for three months until
Galileo's death. Subsequently he was professor to the Florentine Academy
until his death in 1647. Torricelli deduced from the fact that water would
only rise about 32 ft. in a suction pipe, that the air had weight, and could,
therefore, exert a definite pressure equivalent to a height of 32 ft. of water or
2£ ft. of mercury. This conclusion was confirmed by the well-known
Torricellian experiment.
Trace of Rain.—The word " trace " is entered in the daily record sheet
when some rain (or other form of precipitation) is known to have fallen and the
amount in the gauge is less than that required to give a definite entry. If the
gauge contains less than -01 in., but more than half the amount (-005 in.),
the measurement should be entered as -01 in.
The word " trace " is entered :—
(1) When, although there is no water in the gauge, the observer knows
definitely from his own observation that some rain (or other form of
* Hubert: " Mission Scientifique au Soudan." Paris, 1916, pp. 169 and 221.
t De Courcy Ward: "The tornadoes of the United States as climatic phenomena."
London, Quart. J. R. met. Soc., 43, 1917, pp. 317-29.
197
TRADE WINDS
precipitation such as snow, hail, sleet, drizzle) has fallen since the preceding
observation. This happens sometimes, especially in dry warm weather,
without the gauge being even damp.
(2) When the observer finds less than -005 in. of water in the gauge and
he knows definitely that this trace of water is not the result of a drop or two
draining from the sides of the can after emptying the rain water out of it at a
previous time of observation.
Trade Winds.—This is the name given to the winds which blow from the
tropical high pressure belts towards the equatorial region of low pressure,
from the NE. in the northern hemisphere and SE. in the southern hemisphere.
The name originated in the nautical phrase " to blow trade," meaning to
blow in a regular course or constantly in the same direction, afterwards
shortened to " trade." The word is allied to the words " track " and
" tread " and its use in the sense of commerce was a later development of
meaning not implied in its original application. The monsoons were formerly
grouped with the true trades under the same name. Other names were also
given to the trade winds either as a whole or locally. Of these the term
" general wind " has survived in some countries, as for example, the modern
Spanish " vientos generates." Columbus discovered the regularity of the
NE. trade in his first transatlantic voyage in 1492, and this is usually accepted
as the discovery of the trade winds, though previous navigators must have
encountered them to some extent, probably as far back as the time of the
Phoenicians. The Spaniards soon learned the value of the trade winds and
in latter years the galleons regularly utilised the NE. trade of the Pacific in
their journeys from Acapulco (Mexico) to Manila. More or less detailed
descriptions of the trades are extant, dating back to the sixteenth century,
and knowledge of their characteristics was gradually accumulated by mariners.
Lieutenant M. F. Maury, U.S.N., was the first to treat the trade winds
statistically, which he did by means of data collected from ships on a uniform
plan. The results were included in his " Wind and Current Charts " and
" Sailing Directions," published about the middle of the nineteenth century.
As a result of Maury's work, the most important part of which consisted in
finding the quickest sailing-ship routes through the Doldrums, passages were
very materially shortened.
Excluding the Indian Ocean north of the equator, which is subject to
the regime of the monsoons, the trade wind belts occupy a region extending
over 1,000 to 1,400 miles of latitude in all oceans both north and south of
the equator. The average direction of the wind is not the same in different
longitudes of the same ocean. Thus in the North Atlantic, proceeding from
the eastern to the western side of the ocean, the trade wind veers from NE.,
or even from NNE. or N. in some months, through ENE. to E., or to a point
slightly S. of E. In the southern hemisphere corresponding differences are
found, the changes being from S. or SSE., through SE. to E., or even a little
N. of E. The trade winds represent the circulation of the air on the eastern
and equatorial sides of the great permanent oceanic anticyclones which are
found in latitudes of about 30° N. and S. It is interesting to note that the
western margins of the anticyclones are in all oceans much less clearly marked
than are the eastern margins. The easterly winds on their equatorial sides,
such as those of the Carribbean Sea, are really part of the equatorial flow
and the name of " intertropical flow " has been suggested for them. It is
the usual custom to call all the winds between these high-pressure areas and
the Doldrums by the name of trade winds, but the really characteristic trades
are the flows from the eastern sides of the anticyclones, which are, roughly
speaking, NE. in the northern hemisphere and SE. in the southern hemisphere.
The trade winds are not, as is popularly supposed, perfectly steady and
regular either in force or direction. They vary, within limits, from day to
day, from season to season, and from year to year. This applies to all oceans,
but the Atlantic trades are more regular than those of the Pacific, and the
TRADE WINDS
198
SE. Atlantic trade is more regular than the NE. Atlantic trade. Following
the seasonal changes in the sun's declination, the trade winds, together with
the Doldrums and the region of light and variable wind on the poleward
side of the trades, move on the average northward and southward. There
is, however, a considerable time lag behind the sun, thus the NE. Atlantic
trade does not reach the most northerly latitude until August. The rate of
motion is greatest just after the equinoxes. The annual range varies in
different oceans, different longitudes and different hemispheres, but averages
about 5° of latitude. Usually the total width of the belt fluctuates, the
motions of the north and south sides not being the same. In the Atlantic
and Pacific Oceans the SE. trade crosses the equator during the northern
summer, but the NE. trade never extends south of the equator. In the
Indian Ocean the SE. trade never crosses the equator. The day-to-day
fluctuations of the trade winds have their origin in the day-to-day variations
in the position and intensity of the oceanic anticyclones, which affect not
only the force and direction of the trades but also the limiting latitudes.
A table showing the strength of the Atlantic trades is given below.
Average monthly strength of the trades of the Atlantic in miles per hour
b
rt
3
*—)
NE. trade
SE. trade
St. Helena
10
12
14
:ptember
X
%
3
tH
.0
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fc
11
13
13
1
nl
S
11
12
13
•4^
=2
P.
^
12
13
12
S
1
•=f
11
13
11
10
13
12
9
11
12
3
I
<
O)
7
13
15
8
13
17
!
ovember
u
O
£
O
6
12
15
8
13
16
10
13
15
ecember
Year
9-4
12-6
13-8
The St. Helena observations made at an altitude of 1,960 ft. have been
reduced by 15 per cent, for the purposes of the table, which it is considered
gives the velocities that would have been recorded at sea level. The other
figures are derived from Captain Hepworth's investigation of the NE. trade
in lat. 10° N. to 30° N., between long. 30° W. and the west coast of Africa,
and of the SE. trade in lat. 0° to 20° S., long. 0° to 10° W., and lat. 10° to
30° S., long. 0° to 10° E. It should be noted that the coast of Africa disturbs
the regularity of the NE. trade in the eastern part of the area selected. In
the above table, Captain Hepworth's results have been reduced by a little
more than a mile per hour to bring them into conformity with the conversion
values of the Beaufort wind numbers now in use by the Meteorological Office.
It will be seen that the St. Helena winds are appreciably stronger than
the SE. trade marine observations and show a much more definite seasonal
variation. No satisfactory explanation of this fact has been given. The St.
Helena and NE. trade observations are almost exactly complementary ; thus
September, the month of weakest NE. trade, is the month of strongest wind
at St. Helena and so on. Maury derived a ratio of the strength of the NE.
and SE. Atlantic trades from the observed speeds of sailing ships, and this
ratio (6 to 8-8) is exactly the same as the ratio NE. trade/St. Helena for
the year in the above table and does not differ greatly from the ratio
NE. trade/SE. trade. Seasonal variation is seen to be marked in the case of
the NE. trade.
As in the Atlantic, the NE. trade wind of the Pacific lies to the west of
and not opposite to theSE. trade wind, on account of the similarity of curva­
ture of the eastern coast lines of these two oceans. No definite figures
can be quoted for the strength of the Pacific trades, but they are certainly
less than those of the Atlantic, and the relative strengths of the NE. and
SE. trades are reversed, the SE. Pacific trade exhibiting least strength and
steadiness. The SE. trade of the Indian Ocean is most stable when the SW.
monsoon is blowing north of the equator from May to September.
199
TRADE WINDS
Comparatively little exact information is available as to the diurnal varia­
tion of the trades. Galle found that the SE. trade of the Indian Ocean reaches
a maximum at night from May to October, but observations at Porto Rico
and St. Helena give day maxima, less marked at the latter island. There
are many local wind modifications of the trades on continental coasts or
among islands. An interesting feature of the NE. trade is the very large
area of dustfall from the Canary Isles to south of Cape Verde, extending as
far as longitude 40° W.
The explanation of the origin of the trade winds which is given in all
books on physical geography is due originally to a paper published by
Edmund Halley in 1686. It attributes the flow of air northward and south­
ward on either side of the equator to the replacement of air which has been
heated by the warmth of the equatorial belt and has, in consequence,
ascended. Halley's explanation of the easterly component was not satis­
factory, and John Hadley gave the true explanation of this in 1735 by
bringing the effect of the earth's rotation into account. Halley also developed
the theory of the return upper currents, the " counter-trades," or ANTI­
TRADES, blowing from SW. in the northern hemisphere and NW. in the
southern hemisphere. At the present time the only satisfactory explanation
of the trades and counter-trades is to be found by associating these winds
with the pressure distributions obtaining at the surface and at a suitable
height in the upper air. Thus, as has been stated, the surface trades form
part of the circulation round the great oceanic anticyclones. The untenability
of Halley's theory is seen by considering the winds at Suva, Fiji (lat. 18° S.,
long. 177° E.). Here the frequency of SE. wind is only about 10 percent.
Winds from all directions are experienced, that from NE. being the most
frequent. Yet the situation of the island in respect to the equatorial belt of
high temperature is similar to that of St. Helena, where the SE. Atlantic
trade blows steadily. The greater part of the air brought to equatorial regions
by the trades turns eastward and passes away round the western boundaries
of the oceanic anticyclones. Some part of it may go to feed the rainstorms
of the Doldrums.
Direct upper-air observation has shown that the height to which the
NE. Atlantic trade attains is normally little over 3,000 ft., but very great
yariations are found to occur, and it may attain the height of 13,000 ft.
It blows sometimes at the summit of the Peak of Tenerife (12,160 ft.).
Similar fluctuations have been found in the height at which the counter­
trade is met which has been observed to vary from 6,000 ft. to 30,000 ft.
The counter-trade of the North Atlantic Ocean is associated with the high
pressure area which exists over the western Sahara at altitudes of 6,000 ft.
and upwards. The counter-trade of the South Atlantic is similarly associated
with a high-pressure area at this height over southern Africa. The normal
directions for the counter-trade, proceeding northward over the region of
the NE. trade, are successively SE., S. and SW., finally becoming westerly
over the region of the Azores. For the SE. trade they are successively NE.,
N., NW., finally becoming westerly. These directions are confirmed by such
observations as are available. The counter-trades therefore form connecting
links between the upper easterly current over equatorial regions and the
upper westerly current of temperate latitudes. Over the western parts of
the oceans the upper winds over the trades are easterly. This part of the
circulation has a much greater area in the Pacific, where the surface oceanic
anticyclones lie much closer to the eastern shores than they do in the Atlantic.
The weather in trade-wind regions is normally fine and quiet, though four
of the six areas in which tropical cyclones occur lie within their boundaries.
The typical cloud is the detached trade cumulus but ROLL CUMULUS is of
frequent occurrence. There is normally an increase of wind with height
above the sea up to a point which Cave in 1909 at Barbados found to be
about 1,600 ft. Temperature inversion and low humidity have been found
above the trade cumulus in the Atlantic and Indian Oceans.
TRAJECTORY
200
Trajectory.—The path traced out by a particle of air in its movement over
the earth's surface.
If the wind arrows around an ANTICYCLONE are examined they will be
found to suggest a spiral outward motion of the air, while in a DEPRESSION
the spiral motion is inward towards the centre. Thus, if an anticyclone
and a depression remained stationary in close proximity for a sufficient
period of time a particle of air might be found which after rotating around
the anticyclone in the outward spiral was drawn into the depression, its
path finally leaving the earth's surface, as the air rose at the centre of the
latter system. A path of this kind used to be regarded as typical of air
movements associated with anticyclones and depressions. The fact was
overlooked that a depression itself as a system normally moves with a velocity
about as large as that of the individual air particles in their rotation round
it and that before a particle has had time to make a complete rotation, the
system will have moved away and the particle no longer remain under its
influence.
Shaw and Lempfert in the "Life History of Surface Air Currents", a
publication of the Meteorological Office issued in 1906, examined the matter,
by tracing the actual motion of air along the surface as deduced from selected
series of hourly maps and by noting the progress of the air from map to map.
An examination of their charts will reveal how inadequate the older idea was
to express the true trajectory of a given particle in a depression. As the
distribution of cloud and precipitation in a depression is dependent upon the
properties gained by the air within it in the course of that air's travel, the
importance of the study of air trajectories becomes manifest.
During the years that have elapsed since the publication of the " Life
History of Surface Air Currents " much further work has been done on the
subject, and the Norwegian school of meteorologists, especially, emphasise
the importance to the forecaster of the source of any given portion of air,
whether polar or tropical, and of the subsequent travel of that air.
Tramontana.—A local name in the Mediterranean foi a northerly wind.
It is usually dry and cool.
Transparency.—The capacity for allowing rays of light or some other
form of radiation to pass. Thus glass is transparent for the visible radiation
of light. Rock-salt is specially transparent for the rays of radiant heat.
See VISIBILITY and DIATHERMANCY.
Tremor.—See EARTHQUAKES.
Tropical Air (formerly equatorial air).—Air originating in low latitudes,
normally about 30° N., occasionally even further to southwards (see AIR
MASS). Tropical air which reaches the British Isles generally originates in
the Atlantic, and when it reaches our coasts it is characterised by stability
in the lowest few thousand feet, usually with low stratus clouds and often
fog or drizzle. It is modified as it moves inland, especially in summer, when
fair but close weather readily develops from it.
Occasionally tropical air is of continental (African) origin, and if it comes
up over France it gives fine and mild (in summer very warm) weather.
The WARM SECTORS of most depressions consist of tropical air. A current
composed of tropical air is called a tropical current.
Tropical Cyclone.—See CYCLONE.
Tropopause.—The name given by Sir Napier Shaw to the upper limit of
the troposphere.
Troposphere.—A term suggested by Teisserenc de Bort for the lower
layers of the atmosphere. The temperature falls with increasing altitude
up to a certain height (see LAPSE), and the part of the atmosphere in which
Plate V.
to face Page 201
MARCH 29,1927, 7h.
TROUGH OF Low PRESSURE
(FORMERLY V-SHAPED DEPRESSION)
201
TURBULENCE
this fall occurs is called the troposphere. In these latitudes (50°) it extends
from the surface for a thickness of some 7 miles, or 11 kilometres ; in the
tropics the thickness may reach 10 miles, or 16 kilometres.
Trough.—The trough line of a circular DEPRESSION is the line through
the centre perpendicular to the line of advance of the centre.
During the passage of a depression over any given place the pressure
at first falls and later rises ; the trough line passes over the place during the
period of transition from the falling to the rising barometer. On the southern
side of a depression north of the equator, the passage of the trough is
frequently very well marked by phenomena of the type of a LINE-SQUALL,
a sudden rise of pressure takes place, the wind veers, the temperature drops
and there are frequent squalls of rain or hail; the phenomena observed
really indicate the passage of a COLD FRONT.
The word trough is widely used in a more general sense than that given
above for any " valley " of low pressure, and is thus the opposite of a RIDGE
of high pressure.
There is usually (not invariably) a FRONT along the trough. If it is sharply
defined, it corresponds to the old V-SHAPED DEPRESSION. Plate V shows
such a trough; it is an occlusion with warmer surface air behind it.
Tundra.—Treeless lands of northern Canada and Eurasia which lie mainly
just inside or just outside the Arctic circle. In these regions the monthly
mean temperature is below the freezing point most of the year and very low
minima occur in winter. During 2 or 3 months in summer, however, the
average monthly temperature rises above the freezing point and is between
40° and 50° F. in the warmest month, when occasional high temperatures may
be experienced during the daytime. Nevertheless the ground a foot below
the surface remains frozen solid. On the slopes the surface water drains away,
but much of the flat ground becomes marshy. At this season a stunted shrubby
or grassy vegetation grows.
Turbulence.—The irregular or " eddy " motion which appears in fluids,
whether liquid or gaseous, when they flow past solid surfaces or when
neighbouring streams flow past or over each other, provided the velocity
of flow is greater than a certain limit. The distinction between " laminar "
flow and " turbulent " motion in tubes was first demonstrated experiment­
ally by Osborne Reynolds in 1883.
There is no mathematical definition of turbulent flow ; fluid motion is
said to be turbulent when it is impossible to specify the details of the motion
and we have to be content with a knowledge of the mean flow. It is, however,
possible to form a reasonable picture of the effect of turbulent motion on the
properties of the atmosphere, much as it is possible to trace the effects of
the molecular motion of a fluid without tracing the motions of individual
particles.
The motion of the atmosphere is usually turbulent, at least in the lower
layers. The WIND consists of a series of GUSTS and lulls, but the degree of
turbulence present fluctuates considerably, owing to the opposing effects of
solar and terrestrial RADIATION, and to the varying " roughness " of the
ground. Solar radiation promotes the generation of turbulence by day by
setting up an unstable state in the lowest air layers, indicated by the presence
of a high LAPSE RATE ; at night turbulence tends to be damped out whenever
the flow of outgoing radiation can cool the earth sufficiently to set up a large
inversion of temperature near the ground. The " roughness " of the ground
is due to obstacles of such different magnitudes as grass, trees, houses and hills.
(90565)
G*
TURBULENCE
202
Mathematical methods were first applied to the discussion of the effects
of turbulence in the atmosphere by G. I. Taylor,* who showed that the
effects of eddies might be regarded as endowing the atmosphere with a
coefficient of viscosity and a coefficient of heat conduction far greater than
the ordinary coefficients of molecular viscosity and conduction. Taylor
showed that the components of the eddying velocity in three dimensions,
horizontally down wind, horizontally cross wind, and vertically, were roughly
equal. This yields an equipartition in three dimensions of the energy associated
with the eddies, much as the energy of molecular motion shows equipartition.
So far as the equipartition between the horizontal components is concerned,
a test can be made by means of the records of wind velocity and direction
from a pressure-tube anemometer. It can be readily shown that if we take a
portion of the ribbon-like trace shown on the velocity chart, and measure
V. max. and V. min., the velocities in the gusts and lulls respectively, and
if 6 is the angular width of the corresponding direction trace, then the
condition that the down wind and cross wind components shall be on the
average equal is
V. max. — V. min.
. 6
V. max. + V. min.
2
Taylor showed that, on the average, this condition is fairly closely fulfilled.
In Taylor's discussion the effects of turbulence are stated in terms of a
coefficient K, which has the value %wd where w is the mean vertical velocity
in the eddy, and d is the diameter of the eddy. K is the coefficient which
replaces the ordinary molecular coefficient of kinematic viscosity or the
coefficient of thermal conduction, in the ordinary equations of motion or
of heat transfer. Taylor gave the variation of wind with height in terms of
his coefficient K and showed that the calculated distribution is in reasonable
agreement with the observed facts.
The decrease of wind velocity as the earth's surface is approached is a
measure of the rate at which the momentum of the air is being destroyed at
the surface. To keep matters steady there must be a continual downward
flow of momentum, brought about by an exchange of air masses in the vertical,
i.e. by turbulence. Such an exchange must simultaneously cause a transfer
of heat and mass, and of contents of the air mass such as WATER VAPOUR
and impurities—hence the association between inversions of TEMPERATURE
and FOG (see for example the work of O. G. Sutton on the rate of evaporation
from the water surface).
The result of applying the classical equations of DIFFUSION, heat con­
duction and viscous motion to problems of turbulence with little or no change
save in the order of magnitude of the corresponding coefficients did not prove
entirely satisfactory. In the kinetic theory of gases the value of the diffusion
coefficient depends, among other things, on the length of the mean free path
of the molecules, and in 1925 Prandtl introduced for turbulent fluids the
conception of a " mixing length " (Mischungsweg) analogous to the molecular
free path, but with the difference that the mixing length depends on the
position with respect to the boundary. In the atmosphere the problem is
complicated by the varying roughness of the ground and by the changing
stability of flow ; these factors have been investigated by Rossby, Sverdrup
and Paeschke.
It is also necessary to distinguish between small scale phenomena, such
as the wind flow over a small sheet of water or the diffusion of smoke from a
factory, which can be considered as taking place within a relatively thin
boundary layer, and large scale phenomena such as the approach of the
surface wind to the GRADIENT WIND, which are controlled by the permanent
boundary layer, some 2,000 ft. deep, of the earth's atmosphere.
* " Eddy motion in the atmosphere."
London, Philos. Trans., A. 215, 1915, pp. 1-26.
TWILIGHT ARCH
203
In discussing the vertical transfer of heat by turbulence, account must
be taken of the lapse-rate. Heat is transferred in the direction from high
potential temperature to low potential temperature. When the lapse-rate
is less than the adiabatic turbulence transfers heat downwards ; when the
lapse-rate exceeds the adiabatic the transfer is upwards. In either case the
effect of the mixing produced by turbulence is to cause the lapse-rate to
approach the adiabatic.
Twilight.—Twilight is caused by the intervention of the atmosphere
between the sun and the earth's surface. When the sun is some distance
below the horizon the upper layers of air are still illuminated and are reflecting
light to us. The amount of reflected light diminishes as the sun's distance
below the horizon increases, because higher and more diffuse layers alone
are seen in direct sunlight. In the later stages of twilight the light which
reaches us has been reflected more than once.
So early as the 11th century the period of Astronomical Twilight between
sunset and the disappearance of the last trace of daylight was determined as
ending when the sun was 18° below the horizon. Another period, Civil
Twilight, is recognised ending when the sun is about 6° below the horizon
and conditioned by the insufficiency of light for outdoor labour after that
time. Of course both definitions imply skies free from cloud and haze.
The following table shows the duration of astronomical and civil twilight
according to these definitions.
Equator
C .T.
A .T.
Winter solstice
Equinoxes
Summer solstice
h.
1
1
1
m.
16
10
15
h. m.
0 26
0 24
0 26
50°
A .T.
h.
2
1
m
1
52
C T.
h.
0
0
0
m.
45
37
51
A.T.
h. m .
2 48
2 31
•——'
60°
C.T.
h. m.
1
9
0 48
1 59
At midsummer between the Arctic Circle and latitude 48£° there is a
belt with no true night, twilight extending from sunset to sunrise.
The following figures are quoted from Kimball and Thiessen for the
intensity of illumination during twilight with a cloudless sky; they refer
to the illumination of a horizontal surface.
At sunset, 33 foot-candles = 1,600 times light due to full moon in zenith.
Sun 6° below horizon, 0-4 foot-candles = 19 times light due to full moon
in zenith.
Sun 18° below horizon, -0001 foot-candles = -004 times light due to full
moon in zenith.
For comparison the intensity of the illumination due to the sun in the
zenith is given as 9,600 foot-candles.
Twilight Arch.—This term has been used with more than one meaning.
Probably the best practice is to confine it to the boundary between the
illuminated segment and the dark sky above as seen in the later stages of
twilight. In Von Bezold's nomenclature several twilight arches are recognized ;
the first western twilight arch is the boundary seen even before sunset between
the extended tinted band or segment of light along the horizon and the blue
sky above : this arch persists through the development and disappearance
of the " purple light " and may be visible together with the second western
or true twilight arch.
The boundary of the SHADOW OF THE EARTH as seen just after sunset
Von Bezold calls the "first eastern twilight arch". The name twilight
arch is confined to this phenomenon in the fourth issue of the first edition of
the Glossary.
(90565)
G*2
TYPE
204
Type.—Different distributions of atmospheric pressure are characterised
by more or less definite kinds of weather, and accordingly when a certain
form of pressure distribution is seen on a chart the weather is described as
belonging to such-and-such a type. The types are therefore defined by the
shape or general trend of the isobars, thus an " anticyclonic " or a " cyclonic "
type denotes that an ANTICYCLONE or a DEPRESSION is the main feature of the
pressure distribution ; on the other hand a " westerly " type indicates that
the isobars run in more or less parallel lines over a considerable distance from
west to east, having the lowest pressure to the north ; a " northerly " type
will have isobars running north and south with the low pressure to the east,
etc.
The weather associated with each type varies with the season but members
of the same type have nearly always something in common ; thus, an anticyclonic type has usually rainless weather, the cyclonic, wet weather; the
southerly type in the northern hemisphere will in general be relatively warm
and the northerly type cold. The westerly type is a very persistent one and
often gives rise to long periods of rather unsettled weather. The easterly type
gives in winter suitable conditions for severe frosts while in summer in at least
the southern part of the British Isles the weather is usually very warm.
Typhoon.—A name of Chinese origin (meaning " great wind ") applied
to the tropical CYCLONES which occur in the western Pacific Ocean. They
are of essentially the same type as the West Indian hurricane of the Atlantic
and the cyclone of the Bay of Bengal.
" Ulloa's Circle" or " Ring."—A white rainbow or fog bow.
RAINBOW and BOUGUER'S HALO.
[^Ultra-violet Radiation.—See RADIATION.
See
Units.—All physical measurements consist of two parts, (1) a number
and (2) an indication of the magnitude of the " unit " number, i.e. the
magnitude of the measurement if the number became 1. Thus we may say
that a certain article has a length of 10 in. or of 25-4 cm. Here the
units are inch and centimetre respectively, the numbers are 10 and 25-4
respectively, and in view of the relation between the inch and centimetre
the two measurements are consistent.
Much difficulty has arisen in meteorology because the units of length and
of temperature in common use in the British Isles, namely, the inch and the
Fahrenheit degree, differ from those in use on the continent, the centimetre
and the Centigrade degree.
For that reason both British units and continental units are employed in
Meteorological Office publications. The following are the British equivalents
of the more important continental units.
Length
1 centimetre (10 millimetres) = 0-39370 inch.
1 kilometre = 0-62137 mile.
Velocity
1 metre per second = 2 • 23694 miles per hour = 3 • 28084 feet per second
= 1-9435 knots.
1 kilometre per hour = 0-6214 miles per hour = 0-912 feet per second
= 0 • 5396 knots.
1 mile per hour == 0-8684 knots.
Pressure
1 mercury millimetre = 1 • 3332 millibar.
Temperature
Freezing point (32° F.) = 0° Centigrade = 273° Absolute
= 491-4° F. Absolute.
Boiling point (212° F.) = 100° Centigrade = 373° Absolute
= 671-4° F. Absolute.
205
VECTOR
Absolute humidity
1 gramme per cubic metre = 0 • 3596 grains per cubic foot.
Rate of rainfall
1 millimetre per minute = 2-3622 inches pel hour.
See also ELECTRICAL UNITS.
Upbank Thaw.—A state of affairs in which the usual fall of temperature
with height is reversed, a thaw, or an increase of temperature occurring on
mountains, sometimes many hours before a similar change is manifested in
the valleys.
It is due to the superimposition of a warm wind blowing from a direction
differing from that of the surface wind, and occurs most usually at the
break-up of a frost, on the approach of a cyclonic system, but sometimes
during the prevalence of anticyclonic conditions, when a down-current of
air is dynamically heated in its descent from a great height. Under these
conditions, at 9h. on February 19, 1895, at the end of a great frost, the tempera­
ture at the summit of Ben Nevis was 17-6° F. higher than at Fort William,
4,400 ft. below.
This INVERSION of the normal temperature gradient is a contributory
cause of the phenomenon known as GLAZED FROST.
Upper Air.—That part of the atmosphere which is not in close proximity
to the earth's surface. The detailed study of the upper air is an important
branch of meteorological science. The observations of pressure, temperature,
humidity, etc., on which it is based, are obtained in a number of ways involving
the use of aeroplanes, SOUNDING BALLOONS, RADIO-SONDES and radar. The
winds of the upper air are mainly observed by means of PILOT BALLOONS.
V-shaped Depression.—A name formerly used for a sharply denned trough
of low pressure, with the isobars in the form of a V. It is usually a COLD
FRONT or OCCLUSION, more rarely a WARM FRONT. The term is virtually
obsolete, but the expression " V-shaped trough " is occasionally used. Plate V
shows such a trough ; it is an occlusion with warmer surface air behind it.
Valley Breeze.—A breeze which blows during the day up valleys and
mountain slopes. During the day the air in the valley is warmed and
decreases in density; it rises, therefore, and flows up the valley and up the
slopes of the mountain. See ANABATIC.
Vapour Density.—See ABSOLUTE HUMIDITY.
Vapour Pressure.—The pressure exerted by a vapour when it is in a
confined space. In meteorology, vapour pressure refers exclusively to the
pressure of water vapour. When several gases or vapours are mixed together
in the same space each one exerts the same pressure as it would if the others
were not present; the vapour pressure is that part of the whole atmospheric
pressure which is due to water vapour. See AQUEOUS VAPOUR.
Vector.—A straight line of a definite length drawn from a definite point
in a definite direction. A vector quantity is a quantity which has direction
as well as magnitude. In meteorology the wind and the motion of the
clouds are examples of vector quantities ; the directions, as well as the
magnitudes, are required, whereas in the case of the barometer or the tem­
perature, the figures expressing magnitude tell us everything. They are
called SCALAR quantities. All vectors obey the parallelogram law. That
is to say, that any vector A may be exactly replaced by any two vectors
B and C, provided that B and C are adjacent sides of any parallelogram,
and A the diagonal through the point where B and C meet. Also the
converse holds.
The position of an airship, a given time after starting, is an example.
The two vector quantities that bring about the final result are the velocity
and the direction of the airship through the air, and the velocity and direction
of the air, i.e. the wind. Suppose an airship pointing south-west and with
speed of 40 miles an hour. After two hours its position on a calm day is;
80 miles south-west of the starting point. Now suppose the airship has to>
: VEERING
206
move in a S. wind of 30 miles an hour; after two hours this wind alone
would place the airship 60 miles north of the starting point. The real position
will be given by drawing two lines representing these velocities and finding
the opposite corner of the parallelogram of which they form adjacent sides.
See COMPONENT.
Veering.—The changing of the wind in the direction of the motion of
the hands of a watch, in either hemisphere. The opposite to BACKING.
- Velocity.—Velocity is the ratio of the space passed over by the moving
body to the time that is taken. It is expressed by the number of units of
length passed over in unit time, but in no other sense is it equal to this space.
It can be expressed in a variety of units. For winds, metres per second,
kilometres per hour and miles per hour are most common., When a velocity
is variable a very short time is chosen in which to measure it. Thus the
statement " at llh. the wind was blowing at the rate of 60 mi./hr." means
that for one second or so at just llh. the wind had such a rate, that had
that rate continued for an hour, the air would have travelled 60 miles in
that hour.
Vendavales.—Strong squally SW. winds in the Straits of Gibraltar and
off the east coast of Spain. They are associated with depressions and bring
thick rough weather with heavy rain. They blow mainly between September
and March.
Veranillo.—The two or three weeks of fine weather which break the rainy
season near midsummer in tropical America.
Verano.—The long dry season near midwinter in tropical America.
Vernier.—A contrivance for estimating fractions of a scale division when
the reading to the nearest whole division is not sufficiently accurate. The
vernier is a uniformly divided scale which is arranged to slide alongside
the main scale of an instrument. An example of a vernier (on a barometer)
and the method of reading is given in the " Meteorological Observer's
Handbook ".
Virazon.—A Spanish word meaning SEA BREEZE.
Virga.—Wisps or falling trails of precipitation, evaporating before reaching
the ground ; applied principally to ALTOCUMULUS and ALTOSTRATUS.
Virtual Temperature.—The virtual temperature of a sample of air
containing water vapour is that temperature at which completely dry air of
the same total pressure would have the same density as the given sample.
It is easily shown that
T = TI(\-(\-h)elp)
where T is the absolute virtual temperature, T the absolute temperature,
e the water vapour partial pressure, p the total pressure and k the ratio of
the densities of water vapour and dry air at the same temperatuie and
pressure, k = 5/8 nearly.
Viscosity.—The " internal friction " or resistance to distortion which is
exhibited by all real fluids to a greater or lesser extent, and which tends to
dissipate any relative motion of the fluid, the energy lost to the motion
bqing converted into heat. Maxwell (Theory of Heat) defines viscosity as
follows :—" The viscosity of a substance is measured by the tangential
force on unit area of either of two horizontal planes at the unit of distance
apart, one of which is fixed, while the other moves with the unit of velocity,
the space between them being filled with the viscous substance." The
coefficient of viscosity as defined above is usually represented by the symbol
p. In liquids /u decreases with increasing temperature. For water at 0° C.
ft = -0179, while at 100°C./z = -0028. For air at 0°C.^ = 172 x 10~ 6, and
at 100° C. (JL = 217 x 10-«. If p be the density of the fluid, v = /i/p is called
the kinematic viscosity. The kinematic viscosity of water is less than that
of air.
207
VISIBILITY
A very viscous fluid is distinguished from a soft solid by the fact that
any disturbing force, however small, will produce a sensible effect provided
that it is continued for a sufficiently long time. See TURBULENCE.
Visibility.—A term used in describing the transparency of the atmos­
phere and defined by the maximum distance at which an object can be
seen and the clearness with which its details can be discerned. From practical
considerations the convention has been adopted that an object is regarded as
visible only when it can be recognized by an observer who has a prior knowledge
of the character of the object through having seen it on occasions when the
<
atmosphere was clear.
a direct
not
and
quality
composite
a
Strictly speaking " visibility " is
measure of atmospheric transparency. The transparency t may be expressed
as the proportion of light transmitted through unit distance of the atmosphere.
It is simpler in practice to use the obscuring power n of the atmosphere.
The unit of obscuring power is the " nebule " which is defined by E. Gold
as the condition that 100 nebules reduce the intensity of light to 1/1000 part
of its initial value. The number of nebules in a length of 1000 m. of air
varies from 1 in conditions of excellent visibility to about 10,000 in a very
dense fog in which the distance of visibility is only 10 m.
For the same atmospheric transparency the distance of visibility may
vary between rather wide limits and in the case of visibility of lights at night,
the distance increases with the intensity of the light used and with the
darkness of the background. It would therefore be scientifically more
appropriate to specify the transparency as the fundamental quantity, and
to derive from it the visibility under different conditions of illumination or
intensity of light and darkness of background. But the composite quality
has practical advantages because (a) it can be observed with fair approximation
in a simple direct way without much technical training or instrumental
equipment. Both would be necessary to give reasonably accurate measure­
ments of transparency and illumination in the daytime ; (b) it gives the
recipient the information which he wants in the form in which he wants it.
Visibility depends chiefly upon the amount of solid or liquid particles
held in suspension by the air and to a smaller extent upon the uniformity of
temperature and humidity in the layers of air through which an object is
viewed. Visibility may vary very considerably in different directions. On
a sunny day visibility is usually better, i.e. objects can be seen more clearly,
when looking away from the sun than when looking towards it. On a cloudy
day, provided rain is not falling, it is frequently equally good in all directions.
In a general way visibility is usually good in air which originates in
high latitudes and which has had a relatively direct track while moving
towards the equator. Where the equatorial movement is less rapid the
visibility tends to deteriorate as the air moves southwards. On the other
hand air which originates in lower latitudes and moves polewards usually
has less good visibility than air originating near the poles.
The visibility of objects on the ground, when looked at from above, is
sometimes bad even when the visibility between two points on the ground
is good. This condition is due to a layer of haze at a definite height above
the ground. Haze is due to the presence of solid particles in the air and is
often due to large towns. A light north-easterly wind sometimes carries a
smoke haze 70 miles south-west of London while even longer tracks have
been noted to leeward of large industrial areas. The smoke haze has been
found to have a sharply-defined upper level but the height of this surface
above the ground varies.
J. Aitken made measurements with a " dust counter " which determined
the number of hygroscopic particles per cubic centimetre in the atmosphere.
He found values ranging between 16 and 7,000 in the open country and as
many as 48,000 for London. The apparatus used took no account of the
size or composition of the particles.
VOLCANIC DUST
208
More recently, J. S. Owens has used a more sensitive form of dust counter
in which a measured volume of air is drawn through a disc of filter paper
of standard size. The discoloration of this disc is compared with a scale of
shades and the number of particles per cubic centimetre estimated. He has
found in Norfolk with a light NE. wind 100-200 particles per cubic centimetre
with a size of 0-3-1-7 microns, while in London smoke the number may
rise to 50,000 per cubic centimetre. In London in winter with the air fairly
clear the amount of suspended matter is of the order of 1 milligram per
cubic metre of air but it may fall much below this figure on clear nights
between midnight and 6 a.m. During a dense fog the amount may rise
rapidly to 5 milligrams per cubic metre.
In measuring visibility peculiarities of the eye play an important part.
The visibility involves the light emitted by the object and the capacity of
the eye to receive it. The visibility or otherwise of an object depends upon
the contrast between it and its surroundings and when depending upon
differences of illumination, this difference must be a certain percentage of
the total illumination. Differences of form and colour also play a part, the
red fading first as the intensity of illumination is decreased, then the green
and so on until finally a uniform grey is seen.
During the day in non-foggy weather the visibility will usually be decreased
compared with that obtaining in the early morning and near to and after
sunset, on account of turbulence. In regions where there are ascending
columns of air the refractive index of the air at any height will often differ
from that in a descending column of air or air at rest, on account of slight
differences of temperature and density. The light coming to the eye from an
object will be refracted in passing from one column of air to another and
objects are distorted and become indistinct in consequence. This effect is
observed frequently over sandy deserts.
To obtain a numerical measure of visibility a number of objects are
selected at fixed distances. These distances are 20, 40, 100, 200 and 400 metres,
I, 2, 4, 7, 10, 20, 30 and 40 kilometres. The most distant of the selected
objects which can be seen and recognized is noted and its distance away
provides a measure of the visibility. As far as possible objects are selected
which show against the sky line or if this is not possible, those which show
distinctly in contrast with their background.
A simple instrument has been devised recently by E. Gold to measure
the visibility at night. The principle underlying the use of the instrument is
to add to the obscurity of the atmosphere an artificial obscurity by means
of light filters until a known light, when viewed through the filters, becomes
just indistinguishable. The visibility meter consists of a long narrow filter or
screen which is sealed between two glass plates to protect it. The filter
transmits light of all wave-lengths, and its density increases uniformly from
a clear end to a dark end. The glass protector is mounted in a wooden
frame and on this frame is a graduated scale. The frame carries also a
movable piece or slider which contains a short filter also sealed between glass
plates. This filter is precisely similar to the fixed filter but its density
increases in the opposite direction to that of the fixed filter. Thus the combined
density of the two superposed filters is uniform over the area of the movable
filter. The slider has a pointer which moves along the graduated scale and
enables readings to be taken which correspond with the uniform density
of the combined filters. By means of tables the appropriate visibility
corresponding to a specified scale reading can be obtained.
Volcanic Dust.—See DUST.
Warm Front.—The boundary line between advancing warm air and a
mass of colder air over which it rises. The surface of separation or frontal
surface rises from the warm front over the cold air at a smaller angle than
at a COLD FRONT, about 1 in 100 to 1 in 150 being usual figures.
The rising of the warm air over the cold air usually causes considerable
precipitation in advance of the front, while after it has passed there is either
209
WATER
no precipitation or only slight rain or drizzle as a rule. Warm fronts are
chiefly characteristic of high latitudes, and in the British Isles are most often
found in winter. See DEPRESSION.
Warm Sector.—In the early stages of the life history of at least the majority
of the DEPRESSIONS of temperate latitudes, and of more important SECONDARIES,
there is a sector of warm air, which disappears as the system deepens and the
cold front catches up the warm front (see OCCLUSION). The warm sector is
usually composed of TROPICAL AIR, sometimes of maritime POLAR AIR.
Water.—The name used for a large variety of substances such as sea
water, rain water, spring water, fresh water, of which water in the chemical
sense is the chief ingredient. Chemically pure water is a combination of
hydrogen and oxygen in the proportion by weight of one part to eight, or
by volume, at the same pressure and temperature, of two to one, but the
capacity of water for dissolving or absorbing varying quantities of other
substances, solid, liquid or gaseous, is so potent that the properties of
chemically pure water are known more by inference than by practical
experience. They are in many important respects different from those of the
water of practical life. Ordinary water is a palatable beverage, and is a
medium in which a variety of forms of vegetable and animal life can thrive,
but pure water freed from dissolved gases is perfectly sterile and quite
unpalatable.
Ordinary water has a mass of 1 gram per cubic centimetre (62-3 Ibs.
per cubic foot) at 4° C. (39° F.). Sea water contains dissolved salts to the
extent of as much as 35 parts per 1,000 parts of water, and its density varies
from 1-01 to 1-05 gm./cm2.
Rain water falling in country districts is the purest form of ordinary
water; it contains only slight amounts of impurity derived from the
atmosphere. Spring water contains varying amounts of salts dissolved from
the strata of soil or rock through which it has percolated ; the most common,
of these salts is carbonate of calcium, which is specially soluble in water that
is already aerated with carbonic acid gas ; sulphates of calcium and other
earthy metals are also found, and sometimes a considerable quantity of
magnesia. These dissolved salts give the waters of certain springs a medicinal
character. In some districts underground water is impregnated with common
salt and its allied compounds to such an extent that it is no longer called water,
but brine.
When impure water evaporates, the gas that passes away consists of
water alone, the salts, which are not volatile, are left behind ; similarly
when water freezes in ordinary circumstances the ice is formed of pure water,
the salts remain behind in the solution ; so that, except for the slight amount
of impurity due to mechanical processes, pure water can be got from sea water
or any impure water, either by distilling it, or by freezing it.
Besides the solid constituents which give it a certain degree of what is
called " hardness," ordinary water contains also small quantities of gases
in solution, mainly oxygen and carbonic acid. When the water freezes the
ice consists of pure water, and the dissolved gases collect in crowds of small
bubbles.
The thermal properties of water, in the state of purity represented by
rain water, are very remarkable. Starting from ordinary temperatures,
such as 290° A. (62-6° F.) and going upwards in the scale, the water increases
in bulk, and part evaporates from the surface, until the boiling point is
reached, a temperature which depends upon the pressure, as indicated on
p. 116. Then the water gradually boils away without any increase of
temperature, but with the absorption of a great amount of heat. Going
downwards, the volume of the water contracts slightly until the temperature
of 277° A. (4° C., 39-1° F.) is reached ; that is known as the temperature of
maximum density of water. From that point to the freezing point of water,
there is a slight expansion of one eight-thousandth part, and in the act of
freezing there is a large expansion amounting to one eleventh of the volume
of water. It is in consequence of this change of density in freezing that ice
WATER ATMOSPHERE
210
floats in water with one eleventh of its volume projecting, if the ice is clean,
solid ice, and the water of the density of fresh water. Salt water would
cause a still larger fraction to project, but floating ice carries with it a
considerable amount of air in cavities and sometimes a load of earth so
that the relation of the whole volume of an iceberg to the projecting fraction
is not at all definite.
Water Atmosphere.—A general term used to indicate the distribution of
water vapour above the earth's surface. The limitation of the quantity
of water vapour in the atmosphere by the dependence of the pressure of
saturation upon temperature, places the distribution of water vapour on a
different footing from that of the other gases. Water vapour evaporated
at the earth's surface is distributed in the troposphere by convection, which
may cause so much rarefaction and consequent cooling that some of the
water vapour condenses to form cloud.
Watershed.—In physical geography the line separating the head streams
tributary to different river systems or basins, i.e. the line enclosing a
CATCHMENT AREA.
Waterspout.—The term used for the funnel-shaped TORNADO cloud when
it occurs at sea.
Waterspouts are seen more frequently in the tropics than in higher
latitudes. Their formation appears to follow a certain course. From the
lower side of heavy cumulonimbus clouds a point like an inverted cone
appears to descend slowly. Beneath this point the surface of the sea appears
agitated, and a cloud of spray forms. The point of cloud descends until
it dips into the centre of the cloud of spray; at the same time the spout
assumes the appearance of a column of water. The diameter of the pillar is
very variable, estimates ranging from 20 or 30 feet to as much as 200 feet or
more. The height from the sea to the cloud base probably ranges from two
or three hundred feet up to more than 1,000 feet.
The photographs in Figs. 28 and 29 taken from an aircraft represent an
exceptionally fine example which occurred in the eastern Mediterranean off
the coast of Palestine on October 27, 1943, between 0905 and 0915 G.M.T.
The diameter of the pillar estimated from the known length of the ships is
from 200 to 300 feet, the dome of spray about twice this diameter and about
200 feet high, and the height between the sea and the cloud base 1,500 to
2,000 feet. The circulation of the spout was clockwise and it was accompanied
by a definite noise " of rushing wind ". The spout was moving slowly from
north to south ; during the time when it was visible it grew thinner and darker,
and formed a bend which developed into a definite loop.
The duration of a waterspout varies from 10 minutes to half an hour, and
its upper part is often observed to be travelling at a different velocity from its
base until it assumes an oblique or bent form. Its dissolution begins with
attenuation, and it finally parts at about a third of its height from the base
and quickly disappears. The wind in its neighbourhood follows a circular
path round the vortex, and although very local, is often of considerable
violence, causing a rough and confused, but not high, sea. Numerous reports
speak of waterspouts being in the nature of a hollow cylinder of waterdrops
with a more or less transparent central core.
Water Vapour.—See AQUEOUS VAPOUR.
Watt.—The unit of power associated with the practical system of
ELECTRICAL UNITS. It is the rate at which energy is transformed into heat
in a lamp using 1 ampere at 1 volt.
1 watt = 1 ampere-volt = 1 joule per second.
= 107 ergs per second.
1,000 watt = 1 kilowatt = 1£ horse-power.
Units derived from the watt are used by meteorologists for the measurement
of the intensity of radiation.
1 milliwatt per cm.2 = 1 kilowatt per Dm2
= -01435 gram calories per cm.2 per min.
1 ojace page 210.
Photo bv K.A.F.
Fit
28.—Waterspout in the eastern Mediterranean, October 27, 1943, 0910 G.M.T.
View from aircraft at 100 feet, waterspout J—J mile distant.
Photo bv K.A.F.
FIG. 29.—Waterspout in the eastern Mediterranean, October 27, 1943, 0914 G.M.T.
View from aircraft at 500 feet, waterspout \—I mile distant.
211
WAVES OF EXPLOSIONS
Wave Motion.—The commonest example of wave motion is that seen
on the surface of the sea, which is disturbed by gravitational waves of a
sinusoidal form, the waves having a progressive motion perpendicular to
their length. In progressive waves on deep water the water has no steady
iorward motion, and oscillates about .its mean position. Tidal waves are
those in which the motion of the fluid is mainly horizontal and, therefore,
sensibly the same for all particles in a vertical line. Gravitational waves
can also occur in the atmosphere, and Helmholtz suggested that when a
cold current is separated from a warm current by a relatively sharply denned
surface of separation, gravitational waves should form in the surface of
separation. The fundamental feature of wave motion is the alternation of
potential and kinetic energy. Further, the disturbance is in general not
propagated with the velocity of the masses involved in it.
The term wave motion can be applied to a wide variety of vibrating
systems. Waves of compression and rarefaction in the atmosphere are
motions transverse to the direction of propagation of the disturbance. The
period of such a motion is the interval from one maximum of pressure to
the next maximum.
Waves.—Any regular periodic OSCILLATIONS, the most noticeable case
being that of waves on the sea. The three magnitudes that should be known
about a wave are the amplitude, the wave-length, and the period. The
amplitude is half the distance between the extremes of the oscillations, in a
sea wave it is half the vertical distance between the trough and crest, the
wave-length is the distance between two successive crests, and the periodis the time interval between two crests passing the same point. In meteoro­
logical matters the wave is generally an oscillation with regard to time,
like the seasonal variation of temperature, and in such cases the wave-length
and the period become identical.
If a quantity varies so as to form a regular series of waves it is
usual to express it by a simple mathematical formula of the form
y = a sin (nt + a). Full explanation cannot be given here; it must
suffice to say that the method of expressing periodic oscillations by one
or more terms of the form a sin (nt + a) is known as " representing by
a sine curve ", or " a Fourier series ", or as " HARMONIC ANALYSIS ".
Any periodic oscillation either of the air, water, temperature, or any
other variable, recurring more or less regularly, may be referred to as waves.
During the passage of sound waves the pressure of the air at any point
alternately rises above and falls below its mean value at the time. A pure
note is the result of waves of this sort that are all similar, that is to say,
that have the same amplitude and wave-length. The amplitude is denned
in this simple case as the extent of the variation from the mean, while the
wave-length is the distance between successive maximum values. The period
is denned as the tune taken for the pressure to pass through the whole
cycle of its variations and return to its initial value. Another good example
of wave form is provided by the variations in the temperature of the air
•experienced in these latitudes on passing from winter to summer. This
is not a simple wave form because of the irregular fluctuations of tempera­
ture from day to day, and the amplitude of the annual wave cannot be
determined until these ha,ve been smoothed out by a mathematical process.
Fourier has shown that any irregular wave of this sort is equivalent to the
sum of a number of regular waves of the same and shorter wave-length.
In America " heat waves " and " cold waves " are spoken of. These are
spells of hot and cold weather without any definite duration, and do not
. recur regularly.
Waves of Explosions are among the causes which may produce a rapid
variation of pressure which begins with an increase, and is followed by a
considerable decrease. The transmission is in the same mode as that of a
wave of sound. (See AUDIBILITY.) The damage done by a wave of explosion
is often attributed to the low pressure which follows the initial rise. In the
same way the destructive effect of wind is sometimes clue to the reduction
of pressure behind a structure, resulting in the bulging outwards of the
structure itself in its weaker parts.
WEATHER
212
Weather.—The term weather may be taken to include all the changing
atmospheric conditions which affect mankind but by meteorologists it is
more commonly used in a limited sense to denote the state of the sky and
whether rain, snow or other precipitation is falling. Atmospheric obscurity
in the form of fog or mist is also included. In order that the different types
of weather might be recorded concisely, a system of notation was introduced
by Admiral Sir Francis Beaufort which, with some slight modifications and
additions, is in use at the present time. Particulars will be found under
BEAUFORT NOTATION.
Another method, which has the advantage of being independent of
language, of recording the different types of weather concisely and also of
recording optical phenomena is by means of symbols. These have been
agreed to and revised internationally from time to time and Fig. 30 consists
FORMS OF METEOROLOGICAL SYMBOLS
approved by the
INTERNATIONAL METEOROLOGICAL ORGANIZATION, WARSAW, 1935.
(j
PURE AIR
OO HAZE
"7\
V7
SHOWER OF RAIN 8, SNOW
(SLEET)
\/
V
SOFT RIME
SOFT HAIL
V
HARD R1Mt
FOG v<lkm.
A
SMALL HAIL
SHALLOW FOG
A
HAIL
(•)
SUNSHINE
DISTANT LIGHTNING
e
SOLAR HALO
GROUND FOG
f
FROST FOG
DRIZZLE
K
-h
RAIN
-V-
SNOW
-h
DRIFTING SNOW
(HIGH UP)
«——>
V7
DRIFTING SNOW
(NEAR THE GROUND)
GRAINS OF ICE
{If] SNOW LYING
ICl NEEDLES
-^
SHOWER OF RAIN
LUNAR HALO
(J)
SNOWSTORM
v.
DUST DEVIL
GRANULAR SNOW
GALE
THUNDERSTORM
C> DUST OR SANDSTORM
A
GLAZED FROST
SHOWER OF SNOW
^.
MIST
^o
^
SOLAR CORONA
LUNAR CORONA
r\
RAINBOW
AURORA BOREALIS
MIRAGE
ZODIACAL LIGHT
DEW
HOAR FROST
The occurrence of a phenomenon in the vicinity of the station is
Indices 0, 2
denoted by enclosing the symbol within brackets ( ).
may be used to denote intensity.
FIG. 30.
~~
213
WEATHER MAXIM
of a chart showing the symbols approved by the International Meteorological
Organization at Warsaw, 1935.
Weather Map.—A weather map, as the name implies, is one on which
may be found particulars relating to the weather over the area represented
by the map. The particulars may refer to a given instant of time or to the
mean conditions over a period.
A single observer can only observe the weather conditions over a very
limited area ; but by means of an organised corps of observers, synchronous
observations covering a wide area can be made and communicated to a
central office, where they may be used in the construction of a weather map
which is in this case sometimes also called a SYNOPTIC chart.
The meteorological elements most frequently found on a synoptic chart
or weather map are :—the barometer readings, corrected and reduced to mean
sea level, together with the BAROMETRIC TENDENCY ; the direction and
strength of the wind ; the temperature ; the state of the WEATHER indicated
by means of letters and symbols, showing whether it is raining, snowing
or otherwise; the amount and type of cloud and the visibility. Such a
chart when completed presents a bird's eye view of the general weather
situation, and a series of such charts gives the foundations on which our
weather forecasts are based. In order, however, to make a forecast for
any given area, it is necessary for the weather map to include particulars
of the weather distribution both inside and outside the given area. There­
fore, by international agreement, weather information is now distributed
by wireless telegraphy by most countires, some giving the information con­
cerning their own country only and others, in addition, giving a collective
message embracing observations from many countries. It is now possible
to construct daily a weather map based upon observations taken over most
of the inhabited regions of the northern hemisphere.
Weather Maxim.—From the earliest times man has generalised about the
weather with the result that a very large number of weather maxims have
come into existence, the maxims being intended as means whereby the
weather to come may be foretold. A voluminous collection was compiled
and published by Richard Inwards under the title of " Weather Lore."
It may be said at once that only a few of these sayings are true, that a
large number contain only a partial truth, that some are mutually contra­
dictory, and that many are entirely fallacious. This lack of correctness
must be ascribed to the propensity of man to count the " hits " and to
neglect the " misses."
Many maxims concern themselves with the weather that actually occurs
on a specified day and draw conclusions from these conditions as to the
weather during a following period. Of these, a typical case is that of
St. Swithin's Day (July 15) in England, of which it is said—
" If St. Swithin's greets, the proverb says,
The weather will be foul for forty days."
The most cursory examination of this over a few years will amply demon­
strate its unreliability. There is, however, a further point about the saying,
illustrative of the vagaries to be found in weather lore, and that is that
neighbouring nations within a circumscribed area have similar predictions
but, relating to other, and widely separated, days in the calendar. In
France, the day of augury is that-of St. Medard (June 8), in Germany that .of
the Seven Sleepers (June 27), and in Belgium, that of St. Godelieve (July 27).
Other sayings concern themselves with the weather in a part of a given
month, and state that that weather has a controlling influence on another
period. Once again, even cursory examination reveals the lack of truth.
March may come in " like a lion " but that does not mean that it will go
out " like a lamb."
WEATHER REPORT
214
Many other sayings relate to the behaviour of animals. Of these it may
be said that whilst it is highly probable that animals respond to present
conditions, no satisfactory evidence has yet been adduced that they are
capable of forecasting coming conditions.
Other sayings that have gained much currency, concern themselves with
the moon, ascribing to that satellite the power of influencing terrestrial
weather; but this influence has never been demonstrated in an adequate
manner to the satisfaction of meteorologists.
Still other maxims deal with sky colours. Of these, perhaps the best
known is the one—
"A red sky at night
Is the shepherd's delight:
A red sky at morning
Is the shepherd's warning."
which often proves reliable but, on the other hand, quite often proves " a
broken reed."*
In short, no weather maxim should be accepted, no matter how often
it may be quoted, or how traditional, unless it has been subjected to
statistical examination over a long period and has not been found wanting,
or unless it can be definitely connected with specific conditions, as shown
by the weather chart.
Most weather maxims are quite unable to survive the searching tests
implied in the foregoing paragraph. A few of the sailor's sayings, however,
come out better. Of these, three may be mentioned. The saying—
" First rise after low
Foretells a stronger blow "
is often true of the squalls associated with the COLD FRONT of a DEPRESSION.
Similarly, the couplet—
" Long foretold, long last;
Short notice, soon past "
is often true inasmuch as the more extensive depressions with their large
areas of bad weather travel more slowly, as a rule, than do the smaller ones,
and thus their warning signs last longer. Yet again, the—
"A nor-wester is not long in debt to a sou-wester "
is often true, due to the fact that depressions frequently follow one another
closely.
Weather Report.—Statement of the values of meteorological elements
observed at a specified place and time. The elements included depend upon
the purpose for which the report is required.
It is a record of an observation not a forecast.
Wedge.—Between two DEPRESSIONS there is often a region of high
pressure where the isobars are shaped like an inverted V with a rounded
point; as the first depression moves away the pressure rises but as the second
approaches it begins to fall. This region of high pressure is termed a " wedge "
and sometimes a " ridge." It is really a wedge of high pressure between the
two depressions. Since depressions move generally in an easterly direction,
it follows that the motion of the wedge is also generally eastwards. The
weather in the front part of a wedge is often very fine with north-westerly
to northerly winds and very good visibility. When cirrus clouds are seen
they will nearly always be found to be travelling rapidly from the north­
west. On the central part of the wedge the winds are frequently light and
variable in direction, but when this part passes the winds begin to freshen
from the S. or SW., the skies become cloudy, the visibility deteriorates and
rain usually follows as the new depression approaches. When the weather
rapidly clears up after the passage of a depression it may be taken as an
indication of the approach of a wedge and thus of a period, perhaps only
a few hours, of mainly fine weather. (See Plate VI.)
Weighting.—In statistics, varying the share which different figures con­
tribute to some final result in accordance with their reliability or for other
* See London, Met. Mag., 61, 1926 p. 15.
to face Page 2/4
Plate VI
AUGUST 17, 1928, 7 h.
WEDGE
215
WIND
reasons. For example, in calculating the SOLAR CONSTANT from the obser­
vations made at stations of the Smithsonian Astrophysical Observatory,
observations under the most favourable conditions are given a weight of
four, while those only just good enough for inclusion receive a weight of
one. The former are counted four times, the latter only once.
Wet Air.—A term used to define the condition when objects become wet
even when rain is not falling. It occurs when a warm, saturated or practically
saturated air mass replaces a cold dry air mass and is denoted by the letter e
in the Beaufort notation.
Wet Bulb.—A thermometer whose bulb is covered with muslin wetted
with pure water, used in conjunction with a " dry " bulb, reading the air
temperature, as a PSYCHROMETER. The reading of the wet bulb also has some
independent value at temperatures much above the freezing point, for the
discomfort accompanying a given high temperature of the air varies greatly
with the humidity, and is more nearly defined by the wet-bulb reading than
by the temperature of the air. 90° F. is the highest that has been observed
on board ship in the Red Sea. No higher reading is definitely known.
Wet-bulb Potential Temperature.—The wet-bulb temperature which the
air will take up when brought adiabatically to a standard pressure. It has
the property of remaining invariant for ADIABATIC processes, even when water
is precipitated from, or evaporated into, the air mass. It is thus very
valuable for determining the origin of an AIR MASS.
Wet Day.—A wet day is defined for statistical purposes as a period of
24 hours, commencing normally at 9 h., on which 0-04 in. or 1 -0 mm. or more
of rain is recorded. See also RAIN DAY.
Wet Fog.—A fog, accompanied by a very high relative humidity, which
wets objects exposed to it. Many town fogs are due largely to smoke, and
can occur with relatively dry air.
Wet Spell.—The definition of the term " WET SPELL " is analogous to
that of the term " DRY SPELL." Thus a wet spell is a period of at least
15 consecutive days to each of which is credited 0-04 in. of rain or more.
See also RAIN SPELL.
*
Whirlwind.—A small, revolving storm of wind in which the air whirls
round a core of low pressure. Whirlwinds sometimes extend upwards to a
height of many hundred feet, and cause duststorms when formed over a
desert.
Willy-Willy.—The name given in Western Australia to a severe CYCLONE.
Wind.—In the New English Dictionary wind is defined as "air in motion:
a current of air, of any degree of force perceptible to the senses, occurring
naturally in the atmosphere, usually parallel to the surface of the ground."
The motion of air is not restricted by nature to be " parallel to the surface
of the ground " but that is the component of the motion which in ordinary
circumstances is indicated by the weather-cock and by the anemometer.
The definition of wind as the motion of air and the invention of the windvane go back to before the Christian era. It was recognised by the Greeks
that different types of weather were associated with winds from different
directions. The Tower of the Winds at Athens, which is octagonal and dates
back to the 2nd century B.C., carries on its eight sides the names of the winds
and symbolic figures intended presumably to represent the character of the
winds. A description of the figures is given by Theophrastus in his treatise
" On winds and on weather signs " and is quoted by Sir Napier Shaw in the
first volume of his " Manual of Meteorology " together with an estimate of
the corresponding weather.
The habit of naming local winds according to their character or to some
peculiarity associated with the direction from which they blow persists in
the Mediterranean even to the present time. MISTRAL is the master wind
WIND
216
and BORA (north), GREGALE or greco (from the direction of Greece),
LEVANTER and PONENTE (from the direction of the rising and setting sun),
and TRAMONTANA (from the mountains), are all named from their direction.
The habit extends to the Persian Gulf with SHAMAL, nashi, kaus and suhaili
but is less common elsewhere, and in the British Isles HELM WIND is the
only example.
When navigation gradually extended over the whole globe other winds
were soon recognised. The TRADE WINDS which carry air continuously towards
the equator owe their name to the fact that they always keep a steady course.
The ROARING FORTIES or the BRAVE WEST WINDS refer to the persistent
westerly winds which circulate round the globe in about latitude 40° of the
southern hemisphere; and the well-known MONSOON winds which supply
the Indian peninsula with water in the summer are named from their seasonal
character.
Superposed on these winds which form part of the general ATMOSPHERIC
CIRCULATION other local circulations have to be reckoned with. An important
kind of wind is the eddy or vortex in which the air circulates round an axis.
Vortex motion in water is easily visible but for air the motion has to be inferred
from the visible material, leaves or dust, which is carried by the current. On a
small scale the eddies are recognised as a common experience of the effect of
obstacles upon the moving air and are known as GUSTS and LULLS, and on a
larger scale they are experienced as WHIRLWINDS, TORNADOES, tropical
HURRICANES and TYPHOONS and possibly also in some forms of CYLONIC
DEPRESSION though in recent years the vortex theory of cyclonic depressions
has been superseded by the Norwegian theory of the POLAR FRONT.
The approach to numerical expression.— The Beaufort scale.—To give a full
expression of the wind, account has to be taken of its force as well as its
direction. The direction is measured in a horizontal plane and is easily indicated
by the familiar weather-cock which points in the direction from which the air
is flowing past it. In modern times the direction of the wind is reported either
by the point of the COMPASS (true) from which the wind blows, or by its
AZIMUTH in degrees from north through east. A wind which changes its
direction in the same way as the hands of a clock is said to be VEERING and
one which changes in the reverse direction is said to be BACKING.
The observation of wind force offers more difficulty and estimates are based
on the effect of wind on movable objects. Almost anything which is supported
so that it is free to move under the influence of wind can be used. Interest
in the observation was a primary consideration for ships when they depended
upon their sails for their motion and the earliest estimates of the force of the
wind come from the experience of sailors. A scale was first put into numerical
form by Admiral Beaufort and was standardised for use in the Navy in about
1805 (see London, Quart. J. R. met. Soc., 52, 1926, p. 161), but was not finally
adapted for use on land until 1906. A sequence of twelve grades of wind was
enumerated from 0, flat calm, to 12, the force of a hurricane, and is still in use
as the BEAUFORT SCALE.
Speeds greater than that of a hurricane have been recorded on the earth's
surface, and after Sir Douglas Mawson's expedition to the Antarctic it was
suggested that the Beaufort scale should be extended beyond force 12 to
allow for the speeds of the local blizzards to 100-200 mi./hr. The highest
speed recorded on the earth's surface is one of 231 mi./hr. at the Observatory
of Mount Washington at a height of 6,284 ft. on April 12, 1934.
The estimation of wind on a ship at sea is easier than at a station on
land because the evidence provided by the state of the sails can be associated
with the effect of the wind on the water ; and that serves as a reminder
that the surface, either of land or sea, always disturbs the flow of the air,
and part of the wind's energy must be expended in producing the visible
motion of the water. In that respect the sea has great advantages over the
217
WIND
land for the observer because it presents a uniform surface to the moving
air, and in modern times since sailing ships have been replaced by steamers
estimates of wind force at sea depend very largely on the disturbance produced
by the wind on the surface of the water.
The obstacles presented by the land to the air-motion—trees, houses, hills
and other obstructions natural or artificial—have very curious effects upon
the flow of air and the motion often develops into eddies of varying dimensions
(see TURBULENCE). A good specimen is sometimes developed by a strong
wind at a street corner. For this reason the estimation of wind force for
meteorological purposes requires careful consideration.
The measurement of wind ; anemometers and their records.—An instrument
for measuring the force or speed of the wind is called an ANEMOMETER. A form
of pressure anemometer was invented by Sir Christopher Wren in the 17th
century. Until recent years the cup-anemometer, introduced by Prof.
Robinson of Armagh Observatory in 1846, was perhaps the most widely
used of all anemometers. It was put into recording form by Beckley in
1856. The study of winds was carried out at the Royal Observatory,
Greenwich, and at a special establishment of the Meteorological Council at
Holyhead, where various forms of anemometer were installed. After the
Tay Bridge disaster in 1879, special attention was devoted to the measure­
ment of gusts, and a pressure tube anemometer to record the variation in
the force of the wind was devised by W. H. Dines. The testing of anemometers
at the present time is carried out at the National Physical Laboratory at
Teddington, where apparatus is available for creating an artificial wind in a
wind tunnel.
Since the Dines anemometer was brought into use the structure of the
wind near the ground has been the subject of detailed study and continuous
records are now being obtained at some 45 stations in the British Isles.
A study of the records has enabled a distinction to be drawn between GUSTS,
the fluctuations of short period in the force of the wind which are attributed
to the mechanical interference caused by the irregularities of the surface,
and the longer period SQUALLS which are regarded as being of meteorological
origin. The mean hourly wind in the British Isles has rarely if ever exceeded
70 mi./hr., whereas gusts of over 100 mi./hr. have been recorded on several
occasions, the highest authentic record being of 111 mi./hr. at Scilly in 1929.
A remarkable example of a sudden squall was provided by the anemograph
at Kew Observatory on June 1, 1908, when a wind of less than 10 mi./hr.
rose almost instantaneously to nearly 60 mi./hr. in a thunder squall.
Many other notable results are illustrated in a paper by E. Gold on " Wind
in Britain " (London, Quart. J.R. met. Soc., 62, 1936, p. 167), and the results
of a detailed investigation of " The Structure of Wind over Level Country "
in Bedfordshire, based on the work of the late M. A. Giblett, are described
in Geophysical Memoirs, No. 54, 1932.
The practical unit of speed which has been adopted by the International
Commission for Aerial Navigation is the kilometre per hour, and speeds have
been expressed in that unit in the Upper Air Section of the British Daily
Weather Report since January 1, 1938. The metric unit which is in use in
the publications of the Aerological Commission of the International Meteoro­
logical Organization is the metre per second. For countries using British
units, velocities are often given in miles per hour, and sometimes even in
feet per second, and British ships invariably use the knot as a measure of
speed. The equivalents of the various units are as follows :—
1 Km./hr. = -278 m./sec. = -621 mi./hr. = -540 kt. = -911 ft./sec.
The equivalent speeds of the Beaufort forces are given under BEAUFORT
SCALE. For engineering purposes the wind is expressed by the force that it
exerts on unit surface and these equivalents are also included in the Beaufort
table.
WIND
218
The measurement of wind in the free air offers fewer complications than
that at the surface. Various methods have been introduced during the present
century (see PILOT BALLOON, NEPHOSCOPE), and measurements by RADIO­
SONDE are now contemplated. It is important to remember that the measure­
ment of wind obtained by observation of a pilot balloon gives the average
wind over the range of height between consecutive observations and not the
wind at a definite level.
Summaries of surface wind and their representation.—By international
agreement the wind is represented on synoptic charts by arrows pointing
towards the station, with feathers to show the force ; a full-length feather
represents two steps of the Beaufort scale, a short feather one step. This
practice was adopted in the Meteorological Office (except for the Marine
Observer) on March 30, 1936.
The usual method of summarising observations of surface wind for climatic
purposes is to form a table of the frequency of winds from the several points
of the compass. The summaries may be represented pictorially on charts
by means of WIND ROSES.
Instead of a frequency summary it may be an advantage for certain
purposes, such as the planning of air-transport routes, to determine the
resultant wind, either the vector sum of the individual observations or its
components along two specified directions at right angles. Components are
often given along the directions north and east; or for calculating the wind
resistance which a pilot is likely to meet in flying along a definite route,
they may be calculated along and perpendicular to the direction of the route.
Another form of mean wind which is of importance in gunnery is known
as the ballistic wind.
Buys Ballot's law.—In the early days of the development of synoptic
meteorology it was noticed that the flow of air was more accurately described
as being along the direction of the isobars than across them, and that the
closer the isobars were together the stronger was the wind. This relationship
was formulated by the Dutch meteorologist Buys Ballot as BUYS BALLOT'S
LAW and has come to be recognized as one of the fundamental principles of
dynamical meteorology. It has been the subject of a good deal of scientific
work, and with the aid of Coriolis's proposition expressing the relation of
pressure to velocity, which was rediscovered by Ferrel, the law has been
put into mathematical form. A wind which exactly balances the pressure
gradient is known as the GEOSTROPHIC WIND, and when allowance is made
for the curvature of the path of the air as the GRADIENT WIND.
The relationship was found to be much closer over the sea than over
the land, and it was recognized that this was due to the interference of the
earth's surface with the smooth flow of the wind. The various effects that
wind produces locally during its travel, the banging of doors, the raising of
waves, the movement of trees, etc., all dissipate some of its energy by con­
verting it from a steady flow into eddy motion and ultimately into the
irregular motion of heat. In consequence, there is usually a rapid increase
in the speed of the wind with increasing height above the surface until the
undisturbed wind of the free air is reached. The rate of increase depends
upon the degree of mixing or TURBULENCE of the surface and of the upper
layers and is greatest when the mixing is least. A comparison of the wind
in the lower layers of the free air with the gradient by E. Gold in 1908
showed that agreement between the two is usually reached at a height of
about 1,500 ft.
Diurnal variation of wind.—The changes in the degree of turbulence at
the earth's surface have been invoked by Espy and Koppen to explain the
observed variations in the speed of the wind at different times of the day.
At the earth's surface the speed of the wind is greatest by day when the
mixing of the layers of air due to thermal convection brings momentum from
WIND
219
the upper layers to the surface, and is least at night. In the upper air atnga
height of 1,000 ft. the reverse variation is observed, the highest speeds occurri
at night and the lowest during the
day, and at intermediate heights two
maxima are sometimes recorded. The
difference is well illustrated in Fig. 31,
which shows the diurnal variation of
speed in January and July at the top
and bottom of the Eiffel Tower. A
diurnal variation of wind is not
EIFFEL TOWER
likely to be appreciable except in the
it
FT. ABOVE SURFACE
which
1001
to
lowest layers, the height
the
of
speed
the
on
extends depending
surface wind and on the degree of tur­
t——
bulence. The results are set out in
Brunt's " Physical and Dynamical
Meteorology." Very little information
ft
is available about the diurnal variation
e
evidenc
of wind over the sea but such
as there is shows that it is not likely to
be large. On the coasts the diurnal
B.C.M.
variation is complicated by the effect
69FT.ABOVESURfACE
\y
Of LAND AND SEA BREEZES.
There is also a diurnal variation of
wind direction shown as a tendency for
veering during the day and backing at
night, but this is liable to be over-ruled
by local influences.
Variation of wind with height.—The
increase of wind with height is a matter
15 18 tl 24h.
of considerable practical importance.
FIG. 31.—Diurnal variation of wind speed
As a rough approximation it has been
at the top of the Eiffel Tower and at the
wind
the
sea
estimated that over the
Bureau Central Meteorologique (B.C.M.)
at the surface is reduced to about twonear the base.
July ----January ————
thirds of its speed in the free air,
reduced
be
may
it
land
the
over
whilst
to one-third ; though no definite figure applicable to all occasions and places
can be given.
In the immediate neighbourhood of the ground the variation of wind with:
a
height is represented fairly accurately for practical purposes by the formulthe
F100
ft.,
h
Vh= Vwo (a Iog10 h + b) where VH is the velocity at a height
velocity at 100 ft. and a and b are constants. For an open exposure over
level country in the day-time a may be taken as • 335 and b as • 33.
In the layers affected by surface friction a formula for the variation of
wind with height, depending on the coefficient of turbulence, was worked
of
out in 1915 by G. I. Taylor in this country, following the earlier work
been
Ekman, and by Hesselberg and Sverdrup in Norway, and has since the
•elaborated by others. According to the theory the lines representing lie
wind at successive levels are a series of vectors, the extremities of which
on an EQUIANGULAR SPIRAL with its pole at the extremity of the vector
representing the GEOSTROPHIC WIND. The irregularities of the surface, however,
make it difficult to apply the formula in individual cases.
Above the level affected by surface turbulence the wind will approach
the geostrophic velocity provided that the pressure distribution is not changing
rapidly. Hence above about 1,500 ft. the variation of wind with height
depends on the variation of pressure gradient with height, which in its turn
is dependent on the distribution of temperature which changes the atmos­
pheric density and consequently the distribution of pressure.
WIND
220
As a general principle the wind in the free air at the top of any layer
differs from the wind at the bottom by a vector component depending on
the distribution of temperature, which has come to be known as the thermal
wind. In regions where the distribution of temperature does not change
greatly with height, the thermal wind increases with height and its direction
is along the isotherms keeping the low temperature on its left. It follows
from these results, as Brunt has pointed out, that wind will increase with
height if the temperature gradient is in the same direction as the barometric
gradient, i.e. if low temperature is associated with low pressure ; whereas it
will decrease with height if the reverse is the case: If the geostrophic wind
blows from high temperature to low the wind will veer with height, whereas
if it blows from low to high it will back with height.
The magnitude of the thermal wind may be judged from the fact that in
the latitude of the British Isles a temperature gradient of 1° C. per 100 Km.
over a height of 1 Km. will give a thermal wind of 3 m./sec. or 11 Km./hr. ;
or in British units a gradient of 1° F. per 50 miles will give a thermal wind of
4 mi./hr. or 6£ Km./hr. at 3,000 ft. at a temperature of about 30° F.
Causes of wind.—The primary cause of wind has to be traced to differences
in solar and terrestrial radiation setting up irregularities of temperature which
give rise to convection either upwards or downwards. Gravity is the operative
force, working in some cases through the agency of pressure difference.
Water falling on the surface of the earth flows downhill until it comes to
the sea, making room for the water that follows it; similarly on an earth
which was not rotating we should expect the air cooled over high land to
flow down to lower levels. Sir Napier Shaw has given the name KATABATIC
to the winds which automatically flow downhill. Local examples of this
type of wind are not far to seek; in a mild form they are recognised by
every agriculturist and are the cause of the disastrous valley frosts of early
spring ; in a more vigorous form they may be experienced as the fjord winds
of Norway, and on an even larger and more violent scale in the outflowing
winds from Greenland in the northern hemisphere and from the Antarctic
continent in the southern. Katabatic winds are recognised as an important
feature of the meteorology of the Mediterranean, especially in the winter
months ; the BORA and the TRAMONTANA are definitely of that character
and probably the MISTRAL also. These winds occur in the rear of cyclonic
depressions and accentuate the general flow of cold air southwards from the
polar regions which goes to feed the north-east trade winds.
If air is heated at the bottom of a chimney or vertical shaft it will rise up
the shaft if it can be replaced at the bottom by a flow into the shaft, and in
that sense the raising of the air causes the horizontal motion. The creation
of an artificial wind by blowing air with a bellows or air-pump is also a
matter of common experience.
These processes have their analogies in the atmosphere and at one time
were regarded as supplying a sufficient explanation of many of the winds of the
globe. On a small scale the heating of the land by day causes the air to rise and
air from the sea flows in to take its place, and this with the complementary
process at night explained the well-known phenomena of LAND AND SEA
BREEZES ; on a larger scale and with the year instead of the day as the time
unit, the heating of the Asiatic continent in the summer was regarded as
giving rise to an inflow of air towards the continent and so explained the
MONSOONS. Similarly, the air at the equator being warmer than the air of
other latitudes, was regarded as pushed up by the colder air flowing south­
wards and this was held to account for the permanent flow of the TRADE
WINDS.
It was not until the time of Hadley (1735) that it was recognised that the
rotation of the earth itself gave rise to a change of motion which would affect
any moving air in such a way that instead of pursuing its course in a great
circle it would be deviated to the right in the northern hemisphere and to
the left in the southern. This force was used by Hadley to explain the deviation
of the trade winds from due north and south to north-east and south-east.
221
WIND ROSE
For more than a century the explanation was accepted and air was
regarded as flowing from high pressure to low with some deviation caused
by the earth's rotation. Since the time of Buys Ballot, however, pressure
gradient has come to be regarded as the distribution of force which steers the
wind rather than the force which causes it.
In fact, in recent years it has even been suggested that the pressure
gradient may itself be built up by the motion of the air rather than the
reverse ; and there is much to be said for the suggestion. If, for example,
two currents flowing in opposite directions pass one another, each
having the other on the right, account has to be taken of the fact that
the rotation of the earth would interfere with their motion and cause
a displacement of the air to the right and there would thus be a tendency
for the air currents to drift towards each other and hence form a ridge of high
pressure between them ; and in like manner if the flows were reversed so
that the natural effect of the rotation was for the currents to flow away
from one another the result would be that the boundary between the two
would form a trough of low pressure which, with the assistance of local
convection, might lead to the formation of a central area of low pressure
with a cyclonic circulation.
The energy of wind. —A feature of the subject which has received little
attention is the amazing energy of the motion of the air. It has been shown
(Nature, London, 128, 1931, p. 226) that for any part of a horizontal field of
isobars at which the velocity of the air is in geostrophic accord with the
gradient, the energy of a layer of air of uniform thickness, contained within
a square lying between a pair of consecutive isobars, is the same for any part of
the field (due allowance being made for variation of density and latitude) and
can be expressed by the formula | —^cosec2 <j> where h is the height of the
layer, b the isobaric interval, p the density, u> the earth's angular velocity
and <f> the latitude.
Hence the energy of a square column of air 100 m. high between isobars
"with a pressure interval of 2 mb. is about 35 million kilowatt -hours in latitude
60°, and 104 million kilowatt -hours in latitude 30°.
Pressure exerted by wind. —The pressure P in Ibs. per sq. ft. exerted by a
wind of velocity V m.p.h. is customarily expressed as P = -003 V2 .
Wind Rose. —A diagram showing, for a definite locality or district, and
usually for a more or less extended period, the proportion of winds blowing
from each of the leading points of the compass. As a rule the " rose "
indicates also the strength of the wind from each quarter, and the number or
proportion of cases in which the air was quite calm.
The simplest form of wind rose is shown in
Fig. 32 in which the number or proportion of winds
blowing from each of the principal eight points of
the compass is represented by lines converging
towards a small circle, the proportion of winds from
each direction being indicated by the varying length
of the lines. The figure in the circle gives the number,
or percentage, of cases in which the air was calm.
The Baillie wind rose was introduced by
Nav.-Lieut. C. W. Baillie in 1892. In this form of
i form
wind rose, the arrows which fly with the wind show FlG ''
by their length the frequency of winds of various
directions and by their thickness the frequencies of the various forces —light
winds, Beaufort forces 1 to 3 ; moderate winds, 4 to 7 ; and gales, 8 to 12.
The circle supplies a scale for estimating the frequency of winds from any
direction ; the length from the heads of the arrows to the circle measures
5 per cent, of the total number of observed winds (100 per cent. — 2 inches),
the radius of the circle being one-fifth inch or equivalent to 10 per cent, on
WIND VANE
222
the scale. The directions of the observed winds are given to the nearest second
point of the true compass. The upper figure in the centre of the wind rose
indicates the total number of observations upon which it is based and the
lower figure gives the percentage of observations of calms.
Forces
Light. Moderate. Gale.
o
L
Scale
'O
20
30
40
50
<0
70
60
90 "COpercent.
"Total length 2. inches,
which also measure 1O° Long,
on Chart
FIG. 33.—The Baillie wind rose.
A rose may be devised in a similar manner to indicate the relation of
other meteorological phenomena, such as temperature, cloud, etc., to the
direction of the wind.
A wind rose in which the winds for all 12 months are shown on one diagram
was devised a few years ago by Shaw and Garbett.* The eight-point rose is
octagon shaped ; along each side of the octagon are drawn 12 airows indicating
for each month the frequency of light, moderate and strong winds from the
particular direction expressed as a percentage of the total number of winds
observed during the month. The percentage numbers of calms in each of the
12 months are given in the centre. This method of representation has been
adapted for aviation purposes to show on one diagram winds for one month
at different levels in the upper air. The levels selected are those recommended
by the International Commission for Aerial Navigation, viz., surface, 500 m.
above the surface, 1,000 m., 2,000 m. and 3,000 m. above mean sea level.
Thus along each side of the octagon there are five arrows (or columns) indicating
for each level the frequency of winds of different speeds blowing from the
particular direction expressed as a percentage of the total number of winds
observed at that level. The winds are grouped in eight directions and the
ranges of speed in each group in miles per hour are 0-3, 4-15, 16-31, 32-47,
48-62, 63 and over. For a specimen see " Report of International
Meteorological Conference " at Warsaw, 1935 (I.M.O. Publication, No. 29),
Vol. 2, chart at end.
Wind Vane.—A device for indicating the direction from which the wind
is blowing. In the Meteorological Office pattern a horizontal arm pivoted
on a steel spindle is provided with a pointer at one end and an aerofoil of
stream line section at the other. Below the vane is a fixed framework showing
the four cardinal points. If well exposed and accurately balanced, almost
* London, Quart. J.R. met. Soc., 59, 1933, p. 39
223
ZODIAC
any type of vane will show the correct direction in moderate or strong winds,
but ornamental vanes are rarely sensitive enough for meteorological purposes
in light winds.
Winter.—See SEASON'S.
Wireless Telegraphy.—This subject is of interest to meteorologists in many
ways. One of the first practical applications of wireless telegraphy was in
the transmission of weather reports from ships to land. Such reports were
supplied to the Meteorological Office from ships in the Atlantic in 1907.
Synoptic reports summarising meteorological observations over a large area
were first broadcast from the Eiffel Tower in 1911. The interchange of weather
information between the services of different countries is now mostly by
wireless telegraphy. Information for the public is broadcast by wireless
telephony.
The phenomena which interfere with the operation of wireless telegraphy
and telephony and are known as " ATMOSPHERICS " are due to the lightning in
distant thunderstorms.
The transmission of wireless messages to very great distances is affected
by the condition of the higher strata in the atmosphere. It is on account
of the conductivity of these strata, known as the Kennelly-Heaviside layer,
that the electromagnetic waves are able to pass round the globe.
Year.—The time taken by the earth to revolve once in its orbit round the
sun. See CALENDAR.
Zenith.—The point of the sky in the vertical produced upwards from
the observer. The word is now commonly used as denoting a more or less
extensive stretch of sky immediately overhead.
Zenith, Magnetic.—Direction indicated by the upper end of a freely
suspended magnetic needle. In London, for 1937, the direction was 23° southsouth-east of the geographical zenith.
Zephyr.—A westerly breeze with pleasant warm weather supposed to
prevail at the summer solstice.
Zero.—The " point of origin " in graduating an instrument. In Centigrade
and Reaumur thermometers the zero of temperature is taken to be the melting
point of ice, which is accordingly marked " 0 " on the scale. Fahrenheit
took as his zero the lowest temperature he could obtain from mixtures of
ice and common salt. In these cases the zero is an arbitrarily selected point,
and negative values are accordingly possible. Theory indicates that a
temperature below — 273° C. is impossible and that point is accordingly
called the " absolute zero " of temperature, and marked " 0 " on the absolute
scale. An error in positioning the entire scale of an instrument may be looked
upon as an incorrect allocation of the zero and the term " zero error " is
commonly applied to it.
Zodiac.—The series of constellations in which the sun is apparently
placed in succession, on account of the revolution of the earth round the
sun, are called the Signs of the Zodiac, and in older writings give their names
and symbols to the months, thus :—
March is associated with Aries, the Ram.
April
„
„ Taurus, the Bull.
May
,,
,, Gemini, the Heavenly Twins.
June
,,
,, Cancer, the Crab.
July
,,
,, Leo, the Lion.
August
,,
,, Virgo, the Virgin.
September ,,
,, Libra, the Scales.
October
,,
,, Scorpio, the Scorpion.
November ,,
,, Sagittarius, the Archer.
December
,,
,, Capricornus, the Goat.
January
,,
,, Aquarius, the Watercarrier.
February
,,
„ Pisces, the Fishes.
ZODIACAL LIGHT
224
Owing to precession, the position of the equator relative to the zodiacal
constellations has altered a good deal since classical times. The sun now
enters Aries late in April and reaches the other zodiacal constellations with the
same retardation ; but in text books of astronomy the point at which the
sun crosses the equator at the spring equinox, March 21, is still called the
first point of Aries.
Zodiacal Light.—A cone of faint light in the sky, which is seen stretching
along the Zodiac from the western horizon after the twilight of sunset has
faded, and from the eastern horizon before the twilight of sunrise has begun.
In our latitudes it is best seen from January to March after sunset, and in
the autumn before sunrise. In the TROPICS it is seen at all seasons in the
absence of moonlight. The light is usually fainter than that of the Milky
Way. The luminosity is probably due to the scattering of sunlight by an
exceedingly rare gas which is rotating round the sun like an extended
atmosphere. There is no sharp limit to this atmosphere and indeed the
Gegenschein, a luminous patch to be observed in the midnight sky opposite
to the position of the sun is thought to be due to sunlight reflected from
beyond the earth's orbit. According to Jeffreys the apparent luminosity of
the zodiacal light is about 10~ 7 of that of the sky by day. He deduces that the
density of the gas from which the zodiacal light is reflected is of the order
10-is gm./cm3 .
Zone of Silence.—See AUDIBILITY.
Zone Time.—Formerly it was the custom for each country to adopt as
standard of civil time the local mean time of its principal astronomical
observatory ; thus in England, Ireland and France time was regulated by
the local mean times at Greenwich, Dublin and Paris respectively. Conse­
quently Irish time was 25 minutes after English time and French time was
about 5 minutes ahead of English time.
To avoid the confusion so produced zone time was introduced and is
now almost universally adopted. Under this system national standards of
time differ from one another by multiples of half an hour or one hour
corresponding with 7J° or 15° of longitude (see TIME). In countries such
as the United States, which cover vast distances in longitude, two or three
zone times are in use simultaneously. The current practice in respect of
zone time is set out on a chart published by the Hydrographic Office,
Admiralty, and will be found in Whitaker's Almanack.
SATURATED ADIABATIC LAPSE RATES
1000-900
mb.
900-800
mb.
800-700
mb.
700-600
mb.
600-500
mb.
500-400
mb.
400-300
mb.
300-200
mb.
per per
1,000 50
ft. mb.
per per
1,000 50
ft. mb.
per per
1,000 50
ft. mb.
per per
1,000 50
ft. mb.
per per
1,000 50
ft. mb.
per per
1,000 50
ft. mb.
per per
1,000 50
ft. mb.
per per
1,000 50
ft. mb.
•F.
80
70
60
50
2-1
2-3
2-6
3-0
3-1
3-4
3-8
4-3
1-9
2-2
2-5
2-8
3-3
3-6
4-0
4-5
1-9
2-1
2-3
2-6
3-6
3-9
4-3
4-8
40
30
20
10
0
3-3 4-7
3-6 4-9
3-8 5-1
4-1 5-4
4-5 5-7
3-0
3-3
3-6
4-0
4-3
4-7
5-1
5-5
5-9
6-3
3-0
3-2
3-5
3-9
4-3
5-3
5-7
6-1
6-6
7-1
-10
-20
-30
-40
-50
4-7 5-9
4-9 6-0
5-0 6-0
5-1 6-0
4-6
4-9
5-0
5-1
6-5
6-7
6-7
6-7
4-6 7-4
4-9 7-6
5-0 7-7
5-1 7-7
5-3 7-7
Mean
temperature
-60
-70
-80
degrees Fahrenheit
1-9 4-2
2-1 4-6
2-4 5-0
2-3
5-7
2-7
3-0
3-3
3-8
4-2
5-7
6-1
6-5
7-3
7-9
2-5
2-8
3-2
3-6
4-0
4-5 8-4
4-8 8-7
5-0 8-8
5-1 8-9
5-3 8-9
4-4
4-7
4-9
5-1
5-3
..
..
6-2
6-7
7-5
8-2
9-0
2-6
2-9
3-3
3-8
7-5
8-3
9-1
10-4
3-6 12-9
9-6
10-1
10-3
10-5
10-5
4-3
4-6
4-9
5-1
5-2
11-7
12-1
12-4
12-7
12-7
4-0
4-4
4-7
5-0
5-1
5-3 12-7
..
..
..
..
4-i
4-6
4-9
5-1
19-5
21-1
22-1
22-5
3-i ii-3
13-9
14-8
15-4
16-1
16-1
5-2 16-1
5-4 16-1
5-2 22-5
5-3 22-1
5-3 21-7
225
Conversion of degrees Fahrenheit into degrees absolute.
•F.
o
i
2
3
5
4
6
7
8
9
degi ees abso lute
272-4
278-0
283-6
256-3
261-9
267-4
273-0
278-6
284-1
256-9 257-4
262-4 263-0
268-0 268-6
273-6 274-1
279-1 , 279-7
284-7 285-2
258-0
263-6
269-1
274-7
280-2
285-8
258-6
264-1
269-7
275-2
280-8
286-3
259-1
264-7
270-2
275-8
281-3
286-9
259-7
265-2
270-8
276-3
281-9
287-4
260 •:
265 •«
271-:
276 •<
282-4
288 •(
289-1
294-7
300-2
305-8
3II-3
316-9
289-7
295-2
300-8
306-3
3II-9
317-4
290-2
295-8
301-3
306-9
312-4
318-0
290-8
296-3
301-9
307-4
313-0
318-6
291-3
296-9
302-4
308-0
313-6
319-1
291-9
297-4
303-0
308-6
3i4-i
3I9-7
292-4
298-0
303-6
309-1
314-7
320-2
293-0
298-6
304-1
309-7
3I5-2
320-8
293-1
299-:
304-;
310-:
315-1
32i«:
0
255-2
255-8
20
30
40
50
271-9
277-4
283-0
288-6
294-1
299-7
305-2
310-8
316-3
260-8 261-3
266-3 266-9
10
60
70
80
90
100
no
Conversion of barometric height in inches of mercury at 32° F. and latitude 45°
to atmospheric pressure in millibars.
Inches
•00
•01
• 02
•03
•04
•05
•06
•07
•08
•09
mill ibars
28-00
28-10
28-20
28-30
28-40
..
..
..
..
..
948-2 948-5 948-8
951-6 951-9 952-2
954-9 955-3 955-6
958-3 958-7 959-0
961-7 962-1 962-4
949-2
952-6
956-0
959-3
962-7
949-5
952-9
956-3
959-7
963-1
949-9
953-2
956-6
960-0
963-4
950-2
953-6
957-0
960-4
963-7
950-5
953-9
957-3
960-7
964-1
950-9 951-2
954-3 954-6
957-7 958-0
961-0 961-4
964-4 964-8
28-50
28-60
28-70
28-80
28-90
..
..
..
..
965-1
968-5
971-9
975-3
978-6
965-4
968-8
972-2
975-6
979-0
965-8
969-2
972-6
975-9
979-3
966-1
969-5
972-9
976-3
979-7
966-5
969-8
973-2
976-6
980-0
966-8
970-2
973-6
976-9
980-3
967-1
970-5
973-9
977-3
980-7
967-5
970-9
974-2
977-6
981-0
967-8
971-2
974-6
978-0
981-4
968-1
971-5
974-9
978-3
981-7
29-00
29-10
29-20
29-30
29-40
..
..
..
..
..
982-0
985-4
988-8
992-2
995-6
982-4
985-8
989-1
992-5
995-9
982-7
986-1
989-5
992-9
996-3
983-0
986-4
989-8
993-2
996-6
983-4
986-8
990-2
993-5
996-9
983-7
987-1
990-5
993-9
997-3
984-1
987-5
990-8
994-2
997-6
984-4
987-8
991-2
994-6
997-9
984-7
988-1
991-5
994-9
998-3
985-1
988-5
991-9
995-2
998-6
29-50
29-60
29-70
29-80
29-90
..
..
..
..
..
999-0 999-3
1002-4 1002 • 7
1005-7 1006 • I
1009-1 1009 • 5
1012-5 1012-8
999-6
1003 • o
1006-4
1009 • 8
1013-2
IOOO-O
1000-3
1003-7
1007-1
1010-5
1013-9
30-00
30-10
30-20
30-30
30-40
..
..
..
..
..
1015-9
1019-3
1022-7
1026-1
1029-4
1016-2
1019-6
1023-0
1026-4
1029 • 8
1016-6 1016-9
1020-0 1020-3
1023-3 1023-7
1026-7 1027-1
1030-1 1030-5
30-50
30-60
30-70
30-80
30-90
..
..
..
..
..
1032-8
1036-2
1039-6
1043-0
1046-4
1033-2
1036-6
1039-9
1043-3
1046-7
1033-5
1036-9
1040-3
1043-7
1047-1
1003-4
1006-8
IOIO-I
1013-5
1000 • 7 IOOI-O 1001-3 1001-7 1002-0
1004-0 1004-4 1004-7 1005 • i 1005-4
1007-4 1007-8 1008 • i 1008-4 1008-8
1010-8 IOII-2 1011-5 ion -8 IOI2-2
1014-2 IOI4-5 1014-9 1015-2 IOI5-6
1017-3 1017-6 IOI7-9
1020-6 IO2I-0 I02I-3
I024-7
1024-0
1027-4 1027-7 I028-I
I03I-5
1030-8
1024-4
1031-1
1018-3
1021-7
1025-0
1028-4
1031-8
1018-6
1035-5 1035-9
1038-9 1039-3
1042-3 1042 • 6
1045-7 1046-0
1049-1 1049-4
IOlS'9
1022-0 I022'3
1025-4 1025-7
1028-8 IO29 • I
1032-2 1032-5
1033-8 1034-2 1034-5
1037-2 1037-6 1037-9
1040 • 6 1041-0 1041 • 3
1044-0 1044-3 1044-7
1047-4 1047-7 1048 • i
1034-9
I038-2
I04I-6
1045-0
1048-4
1035-2
1038-6
1042 • o
1045-4
1048-7
31-00 .. 1049 • 8 1050-1 1050-4 1050-8 1051-1 1051-5
I05I-8
1052-1 1052-5 1052-8
(90565)
H
226
LIST OP
The following pages give the equivalents in various languages of a number of the terms
identical in all languages, and these have been omitted from the lists. The complete list
ENGLISH
Absorption (atmospheric)
Accumulated temperature
Accuracy
Actinic rays
DANISH
Atmosfaerisk Absorption
Tempera turoverskud
N0jagtighed
Aktiniske Straaler
Afterglow
Eftergl0d
Air
Air-meter
Anemoscope
Anthelion
Anticyclone
Luft
Vindhastighedsmaaler,
Anemometer
Lufthuller
Luft-Str0mningslinje
Hojdemaaler
H0jde
Anabatisk
Anemogram
Vin dstyrkemaaler,
Anemometer
Anemoskop
Modsol (Bisol)
Anticyklon
Antitrades
Anvil cloud
Antipassat
Ambolt Sky
Aqueous vapour
Arid
Atmosphere
Atmospheric pollution
Vanddamp
T0rt
Atmosfaere
Luftens Urenhed
Atmospheric pressure
Atmospherics
Lufttryk
Luftelektriske Forstyrrelser
Waterdamp
Droog, aride
Dampkring, atmospheer
Verontreiniging van de
Atmospheer (dampkring)
Luchtdrukking
Luchtstoringen
Attached thermometer
Paasat Termometer
Aangehechte Thermometer
Audibility
Aurora
H0revidde
Polarlys, Nordlys
Hoorbaarheid
Poollicht, Noorderlicht
Autumn
Avalanche wind
Average
Efteraar
Faldvind
Middel—
Herfst
Lawine-wind
Gemiddelde, middelwaarde
Backing
Vinden drejer til venstre,
dreier mod Solen
Kuglelyn
Ballon
Pejling
Krimpend
Air pockets
Air trajectory
Altimeter
Altitude
Anabatic
Anemogram
Anemometer
Ball lightning
Balloon
Bearing
Beaufort notation
Beaufort's Vejr-Kode
Beaufort scale of wind
Beaufort's Vindskala
force
DUTCH
Absorbite (atmospherische)
Overmaat van temperatuur
Nauwkeurigheid
Actinische (photographisch
werkzame) stralen
Naschemering
Lucht
Windsnelheidsmeter
Remou's
Luchtbaan
Hoogtemeter
Hoogte
Stijgend
Windregistreering
Windmeter
Windaanwij zer
Tegenzon
Gebied van hooge drukkingmaximum
Antipassaat
Aambeeld wolk
Bolbliksem
Ballon
Peiling
Beaufort teekens
Beaufortschaal voor
windkracht
227
EQUIVALENTS
defined in the Glossary. Many of the words were found to be practically
of translations are available for reference in the Meteorological Office Library.
FRENCH
Absorption (atmospherique)
Temperatures accumulees
Exactitude, precision
Rayons actinique
GERMAN
Absorption (atmospharische)
Warmesumme, Temperatursumme
Genauigkeit
Aktinische Strahlen
Lueurs crepusculaires
Nachgliihen
Air
An6mometre a main
Luft
Anemometer, Windmesser
Trous d'air
Trajectoire de 1'air
Altimdtre
Altitude
Anabatique
Anemogramme
Anemometre
Luftlocher
Luftbahn, Trajektorie
Hohenmesser
Hohe
Anabatisch, konvektiv
Anemogramm, Windregistrierung
Windmesser
Anemoscope
Anthelie
Anticyclone
Windfahne
Gegensonne
Antizyklone, Hoch, Hochdruckgebiet
Centre Aliz6s
Nuage en enclume
Antipassat, Gegenpassat
Ambosswolke
Vapeur d'eau
Aride
Atmosphere
Impuretes atmosph6riques
Audibilit6
Aurore
Wasserdampf
Trocken
Atmosphare, Lufthiille
Staubgehalt, Beimengungen,
Verunreinigung der Luft
Luftdruck
Atmospharische Storungen (drahtL
Telegraphic)
Attachiertes (eingefiigtes)
Thermomete
Horbarkeit
Nordlicht
Automne
Vent d'avalanche
Moyenne
Herbst
Lawinenwind
Mittelwert
Mouvement levogyre,
Recul du vent
Eclair en boule
Ballon
Azimut, relevement (terme de
marine)
Notation de Beaufort
Echelle de Beaufort pour la
force du vent
Zuriickdrehen
Pression atmosph6rique
Atmospheriques
Thermometre de barometre
(C0565)
Kugelblitz
BaUon
Peilung
Beauforts Wetterskala
Beauforts Windstarkeskala
H2
228
LIST OF EQUIVALENTS
DUTCH
ENGLISH
Black-bulb thermometer
DANISH
Sortkugletermometer
Blizzard
Blood-rain
Blue of the sky
Brave West Winds
Blizzard
Blod-Regn
Himlens blaa Farve
Brave Vestenvinde
Zwarte-bolthermometer
(stralingsthermometer)
Sneeuwstonn of sneeuwdrift
Bloedregen
Hemelblauw
Westenwind gebied
Breeze
Bumpiness
Buoyancy
Bris, Brise
Ujaevnhed i Luften
Opdrift
Bries (wind)
Onrust van de Atmospheer
Veerkracht
Calibration
Calm
Cap
Catchment area
Clear sky, Day of
Climatic changes
Kalibrering
Vindstille
Skykappe
Nedborsopland
Klar Dag
Klimaaendring
IJking
Windstilte
Kap(je)
Draagoppervlakte
Heldere dag
Klimaatschommelingen
Cloud-burst
Cloudiness
Clouds
Col
Cold front
Cold wave
Compass
Skybrud
Skydaekke
Skyer
Saddel
Kold Front
Kuldeb01ge
Kompas
Wolkbreuk
Bewolking ('s-graad)
Wolken
Zadel
Koud front
Kou-inval. Kougolf
Kompas
Conduction
Corona
Crepuscular rays
Curve fitting
Ledning
Korona
Tusm0rkestraaler
Udjaevning med Kurve
Geleiding
Krans
Schemeringstralen
Kromme aanpassen
Damp air
Dawn
Day
D6bacle
Fugtig Luft
Daggry
Dag
Isbrud, Isgang
Vochtige lucht
Dageraad, Morgenschemering
Dag
IJsgang
Density
Depression
Desert
Desiccation
Deviation
Dew
Diathermancy
Taethed
Lavtryk
0rken
Udt0rring
Deviation
Dug
Gennemstraalelighed
Diffraction
Diurnal
Doldrums
B0jning
Daglig
Doldrum
Dichtheid
Depressie, minimum
Woestijn
Uitdroging
Afwijking
Dauw
Doorlaatbaarheid voor
warmte
Bulging, diffractie
Dagelijksch
Kalmtegordels
Drizzle*
Drops
Drought
Dry spell
St0vregn
Draaber
T0rke
T0rt Tidsram
Motregen, miezeren
Druppels
Droogte
Tijdperk van Droogte
* American equivalent, mist
229
—continued
FRENCH
Thermometre a boule noire
GERMAN
Schwarzkugelthermometer
Blizzard
Pluie de sang
Bleu du ciel
Braves vents d'ouest (terme de
marine)
Brise
Turbulence de 1'air
Force ascensionnelle
Blizzard, Schneesturm
Blutregen
Blaues Himmelslicht, Himmelsblaue
Brave Westwinde
Calibrage
Calme
Capuchon (pileus dans 1'Atlas)
Bassin de reception
Ciel clair, Definition d'un jour de,
Changements ou Variations
climateriques
Pluie torrentielle
Nebulosite
Nuages
Col
Front froid
Vague froide
Boussole, compas
Kalibrierung, Eichung
Kalme, Windstille
Kappe
Einzugsgebiet
Heiterer Tag
Klimaanderung
Conduction
Couronne
Rayons cr^pusculaires
Choix de courbes
Leitung
Korona, Hof
Dammerungstrahlen
Kurvenanpassen
Air humide
Aube
Jour
Debacle
Densit6
Depression
Desert
Desse'chement
Deviation
Ros6e
Diathermansie
Feuchte Luft
Dammerung
Tag
Beginn des Eisgangs (Aufgang der
Flusse)
Dichtigkeit
Depression
Wiiste
Austrocknung
Abweichung, Ablenkung
Tau
Diathermansie, Warmedurchlassigkeit
DiflEraction
Diurne
Pot-au-noir
DifEraktion
Taglich
Doldrums, Stillen, Kalmengurtel
Bruine
Gouttes
S6cheresse
Serie (ou p^riode) s£che
Spriihregen, Nieseln, Rieseln
Tropfen
Diirre
Trockenperiode
Brise
Boigkeit (" Bockigkeit ")
Auftrieb
Wolkenbruch
Bewolkung
Wolken
Sattel (zwischen zwei Antizyklonen)
Kaltfront
Kaltewelle
Kompass
230
ENGLISH
LIST OF EQUIVALENTS
DUTCH
Duration of sunshine
Dusk
DANISH
Varighed af Solskin
Tusm0rke
Duur van Zonneschijn
Avondduister
Dust
Dust-devil
St0v
St0vhvirvel
Stof
Zandhoos
Dynamic cooling
Dynamisk Afk01ing
Dynamische afkoeUng
Earth currents
Earthquake
Eddy
Equation of time
Equatorial air
Equinox
Error ,
Evaporation
Expansion
Exposure
Exsiccation
Extremes
Eye of storm
Eye of wind
Jordstremme
Jordskselv
(1) Str0mskaer (2) Hvirvel
Tidsaekvation
AEkvatorial Luft
Jaevnd0gn
Fejl
Fordampning
Udvidelse
Opstilling
Udt0ning
Ekstremer
Stormens 0je
Vind0je
Aardstroomen
Aardbeving
Draaikolk
Tij dvereffening
Equatoriale lucht
Dag en nacht evening
Fout
Verdamping
Uitzetting
OpsteUing, blootstelling
Uitdroging
Uitersten
Oog van den storm
Hoek waaruit de wind waait
Falling time (of
barometer)
Fiducial temperature
Barometrets Traeghed
Valtijd (van den Barometer)
Referens Temperatur
Standaardtemperatuur
herleid voor breedte
Floe
Flow-off
Fog
Fog bow
Forecast
Frazil ice
Isflage
Afstramning
Taage
Taagebue, Hvid Regnbue
Vejrforudsigelse
Bank van drijfijs, ijsbank
Afvoer
Mist
Mistboog
Verwachting
(Z wevend ijs)
Freeze, Freezing
Frequency
Fryse, Frysning
Hyppighed, Frekvens
Friction
Front
Frost
Funnel cloud
Further outlook
Gale
Gale warning
General inference
Glacier breeze
Glazed frost
Grass temperature
Gravity
Great circle
Green flash
Ground frost
Bevriezen, vriezende
Frequentie (taMjkheid,
veelvuldigheid)
Gnidning, Friktion
Wrijving
Front
Front
Frost
Vorst
Skytap
Ikkestaart
Vejrforudsigelse paa lang Sigt Verdere vooruitzichten
Stonnende Ruling
Stormvarsel
Vejrberetning
Gletschervind
Islag, Isslag
Temperatur i Graesset
Tyngde
Storcirkel
Den gr0nne Straale
Storm
Stormwaarschuwingen
Algemeen overzicht
(Gletscher-bries)
IJzel
Temperatuur op het grasveld
Zwaartekracht
Groote cirkel
Groene straal
Nachtvorst
231
—continued
FRENCH
Duree de 1'Insolation
Brune (obscurite)
GERMAN
Poussiere
Tempdte de sable
Sonnenscheindauer
(Die Zeit vom Ende der biirgerlichen
Dammerung bis zur vollen
Dunkelheit)
Staub
Staubwirbel, Sandhose
Refroidissement dynamique
Dynamische Abkuhlung
Courants telluriques
Tremblement de terre
Tourbillon
Equation du temps
Air equatorial
Equinoxe
Erreur
Evaporation
D6tente
Exposition
Dessechement
Extremes
CEil de la Tempete
Lit du vent (marine)
Erdstrome
Erdbeben
Wirbel
Zeitgleichung
Aquatorialluft
Aquinoktien, Tag- und Nachtgleiche
Fehler
Verdunstung
Ausdehnung
Aufstellung
Austrocknung
Extreme
Auge des Sturms
(Richtung, aus der der Wind weht)
Inertie
Fallzeit (des Barometers)
Die Temperatur, bei der die Ablesungen
eines in mb. eingeteilten Barometers
in einer bestimmten Breite keiner
Korrektion bediirfen
Eisfeld
Floe
Abfluss
D6bit
Nebel
Brouillard
nbogen
Nebelrege
blanc
ciel
Arc en
Vorhersage
Pr6vision
(Intraduisible en francais—Sorte de Siggeis
bouillie de glace)
Frieren
Gel, Congelation
t
Haufigkei
Frequence
Frottement
Front
Gelee
Entonnoir de la trombe
Provision a assez longue echeance
Coup de vent
Avertissement de tempdte
Provision du type de temps
Brise de glacier
Verglas
Temperature du sol gazonn6
Gravite
Grand cercle, orthodromie
Rayon vert
Sol gete
Reibung
Front
Frost
Trichter, Schlauch, Riissel-Wolke
Voraussichtliche Weiterentwicklung
der Wetterlage
Sturm
Sturmwarnung
Allgemeine Wetterlage
Gletscherwind
Glatteis
Temperatur am Erdboden
Schwere
Grosser Kreis
Griiner Strahl
Bodenfrost
232
LIST OF EQUIVALENTS
ENGLISH
Ground, State of
DANISH
Jordoverfladens Tilstand
DUTCH
Grondtoestand
Gust
Vindstod, Vindkast
Windstoot
Hail
Halo
Hagel
Halo (kring)
Haze
Heat
High
Hoar-frost
Horse latitudes
Humidity
Hurricane
Hydrometer
Hagl
Ring, Maanegaard, Solgaard,
Halo
Dis
Varme
H0jtryk
Rimfrost
Heste-Bredder
Fugtighed
Orkan
Hydrometer Areometer
Ice
Iceberg
Iceblink
Ice sheet
Increment
Index correction
Is
Isfjeld, Isbjerg
Isblink
Isdaekke
Tilvaekst
Nulpunktskorrektion
Us
IJsberg
IJsblinker
IJslaag
Vermeerdering, toeneming
Index-correctie
Indian Summer
Indiansk Sommer
Nazomer
Isopycnic
Isopycne
Isopyknen
Katabatic
Kite
Fald (vind)
Drage
Dalend
Vlieger
Labile
Lag
Land and sea breezes
Lapse
i ustabil Ligevaegt
Indstillingstid
Land og Havbris (-vind)
Lodret Temperatur-Gradient
Labiel, wankelbaar
Achterblijven
Land en zeewind
Verval, verloop
Latitude
Level
Lightning
Line-squall
Liquid
Local time
Longitude
Looming
Low
Lunar
Bredde
Vandstand, Niveau
Lyn
Bygelinie
Vaedske
Lokal Tid, Stedtid
Laengde
Ragen op i Taagen
Lavtryk
Maane-
Breedte
Waterpas, vlak, niveau
Bliksem, weerlicht
Bui
Vloeibaar
Plaatselijke tijd
Lengte
Opdoemen
Laag, depressie, minimum
Maans-
Mackerel sky
Himmel med Makrelskyer
Schaapj eswolken
Mares' tails
Mercury
Mirage
Mist
Monsoon
Month
Moon
Mountain breeze
Kattehaler
Kviks01v, Kvaegs01v
Spejling
Let Taage
Monsun
Maaned
Maane
Bjergvind
Paardestaarten (windveeren)
Kwik
Luchtspiegeling
Nevel
Moesson
Maand
Maan
Bergwind
Nevel
Warmte, hitte
Hoogedrukgebied
Ruige vorst
Stiltegordels
Vochtigheid
Orkaan
Vochtweger
233
—continued
FRENCH
Etat du sol
GERMAN
Zustand des Bodens
Rafale
Windstoss
Grele
Halo
Hagel
Halo
Brume seche
Chaleur
Anticyclone
Gelee blanche
Zone des calmes tropicaux
Humidite
Ouragan
Hydrometre
Dunst
Warme
Hoch, Hochdruckgebiet
Rauhreif
Rossbreiten
Feuchtigkeit
Hurrikan
Hydrometer
Glace
Iceberg
Reflet eblouissant de la glace
Couche de glace
Difidrentielle
Correction instrumentale
Isopycniques
Eis
Eisberg
Eisblink
Eisschild
Inkrement
Index-Korrektion, InstrumentalKorrektion
nd
Deutschla
in
Indianer-Sommer,
sommer
Altweiber
mit
eutend
gleichbed
Isopyknen
Catabatique
Cerf-volant
Katabatisch
Drachen
Labil
Equilibre instable
Nachhinken, Tragheit
Retard
und Seewinde (brisen)
Landmer
de
Brise de terre et brise
, Abfall
Abnahme
re
temperatu
la
de
nce
Decroissa
avec la hauteur
Breite
Latitude
Ebene, Hohenlage, Niveau
Niveau
Blitz
Foudre, eclair
Boe (mit breiter Front)
Ligne de grain
Fliissigkeit
Liquide
Ortszeit
Heure locale
Lange
Longitude
Mirage, deplacement de 1'horizon
Tief, Tiefdruckgebiet
Depression
Lunar
Lunaire
Ciel moutonne
Cirrus en queue de cheval
Mercure
Mirage
Brume
Mousson
Mois
Lune
Brise de montagne
(90565)
(Ein mit cc. oder ac. undul
bedeckter Himmel)
Federwolke (Cirrus), Windbaum
Quecksilber
Luftspiegelung, Fata Morgana
Nebel
Monsun
Monat
Mond
Bergwind
H*
234
ENGLISH
Night sky, Light of
DANISH
Nattehimlens Lys
Normal law of errors
Typiske Fejllov
LIST OF EQUIVALENTS
DUTCH
Nachtelijken hemel, licht van
den
Foutenwet
Observer
Oscillation
Overcast day
Observator
Svingning
Overtrukket, Graavejrsdag
Waarnemer
Schommeling
Betrokken dag
Pack ice
Pallium
Pakis
Pakijs
(Pallium)
Paranthelia
Paraselenae
Parhelion
Passing shower
Percolation
Persistence
Personal equation
Phenomenon
Pilot balloon
Pitot tube
Pluviometer
Pocky cloud
Polar air
Precipitation
Pressure
Probability
Probable error
Prognostics
Pumping
Purple light
Bisole
Bimaaner
Bisol
Vandrende Byger
Nedsiven
Uforanderlighed
Personlig Fejl
Faenomen
Pilot Ballon
Pitot R0r
Regnmaaler
Polarluft
Nedb0r
Tryk
Sandsynlighed
Sandsynlig Fejl
Vejrregler, Vejrtegn
Pumpning
Purpurlys
Bijtegenzon
Bijmaan
Bijzon
Losse bui
Doorsikkering
Aanhouden
Persoonlijke fout
Verschijnsel
Loodsballon
Pitotbuis
Regenmeter
(Mammato-cumulus)
Poollucht
Neerslag
Druk
Waarschijnlijkheid
Waarschijnlijke fout
Verwachtingen
Pompen
Purpiirlicht
Radiation
Rain
Rain, Artificial
Rain-band
Rainbow
Rain day
Raindrops
Rainfall
Rain-gauge
Rain-making
Straaling
Regn
Kunstig Regn
Regnbaand
Regnbue
Regndag
Regndraaber
Nedb0r
Regnmaaler
Kunstig Fremstilling af Regn
Rain spell
Recurvature of storm
Regnperiode
Afb0jning af Stormbane
Straling
Regen
Regen, kunstmatige
Regen band
Regenboog
Regendag
Regendruppels
Regenval
Regenmeter
Regenmaken,
regenverwekken
(Regen schaduw,
beschutting)
Regen periode
Terugloopen van den storm
Reduction
Reflection
Refraction
Residual
Reversal
Reduktion
Reflektion
Refraktion
(1) Rest- (2) Afvigelse
Vindomslag med H0jden
Herleiding
Terugkaatsing
Breking, refractie
Achterblijvend, overblijvend
Omkeering
Revolving storm
Hvirvelstorm
Wervelstorm
Rain shadow
235
—continued
FRENCH
Lumidre du ciel nocturne
GERMAN
Licht des Nachthimmels
Courbe en cloche ou Courbe de
Gauss
Observateur
Oscillation
Jour convert (completement
couvert)
Pack ice
Pallium
Fehlergesetz
Paranth61ie
Paraselene
Parheiie
Averse passagdre
Percolation
Persistance
Equation personnelle
Ph6nom£ne
Ballon-pilote
Tube de Pitot
Pluviome'tre
Nuage en poche, Nuage en sac
Air polaire
Precipitation
Pression
Probabilit6
Erreur probable
Pronostics
Pistonnage
Lueurs pourpres
Beobachter
Oszillation, Schwankung
Triiber Tag
Packeis
(Eine gleichemassige, graue Wolkendecke, aus der gewohnlich Regen
fallt)
Nebengegensonne
Nebenmond
Nebensonne
Schauer
Filtrierung, Durchsickerung
Persistenz, Bestandigkeit
Personliche Gleichung
Phanomen
PilotbaUon
Pitot-Rohre
Pluviometer, Regenmesser
(Mammato-cumulus)
Polarluft
Niederschlag
Druck
Wahrscheinlichkeit
Wahrscheinlicher Fehler
Prognose, Vorhersage
Pumpen (beim Barometer)
Purpurlicht
Radiation
Pluie
Pluie artificielle
Bande de la pluie (dans le spectre)
Arc en ciel
Jour de pluie
Gouttes de pluie
Chute de pluie
Pluviometre
Pluie artificielle
Strahlung
Regen
Kiinstlicher Regen
Regenbande (im Spektrum)
Regenbogen
Regentag
Regentropfen
Regenfall, Niederschlag
Regenmesser
Regenmachen
Region abritee de la pluie
Regenschatten
Regenperiode
S6rie de pluie
Umbiegen der Sturmbahn
Courbure de la trajectoire d'un
cyclone
Reduktion
Reduction
Reflektion
R6flexion
Refraktion, Brechung
Refraction
Abweichung
R6siduel
Inversion dans la direction du vent, Umkehrend
renversement du vent
Drehsturm
Tempete tournante
(90565)
H*2
236
LIST OP EQUIVALENTS
ENGLISH
Ridge
Rime
Roaring Forties
Roll cumulus
Salinity
Sand pillar
Sastrugi
Saturation
Scotch mist
Screen, Stevenson
Scud
Sea breeze
Sea disturbance
Sea level
Seasons
Secular trend
Seisms
Serein
Shade temperature
Shower
Silver thaw
Site
Sleet*
DANISH
(1) Ryg (2) Kite
Rim
De bralende Vinde paa 40°
Skyp01ser
Saltholdighed
Sands0jle
Maetning
Stevensons Termometerhus
Drivsky, Stormsky
S0bris, Havbris
S0gang
Havets Overflade
Aarstider
Sekulaer Gang
Jordrystelser
Regn fra en klar Himmel
Temperatur i Skyggen
Byge
Rimslag
Beliggenhed
Slud
Slide rule, Pilot balloon
Pilot Ballon Regnestok
Sling thermometer
Snow
Snow drift
Snow line
Snow lying
Snow rollers
Soft hail
Solarisation
Solstice
Sounding balloon
Spring
Squall
Squall line
Standard
Standard deviation
Standard time
State of the sky
Storm
Storm cone
Subsidence
Summer
Sun-dial
Sun pillar
Sunrise, sunset
Sunrise and sunset colours
Slyngtermometer
Sne
Snefog, Sneknog
Snegraense
Snedaekke
Hagl, Snehagl
Direkte Solstraaling
Solhverv
Ballon sonde
Foraar
Byge
Bygelinie
Norm
Middelfejl
Standard Tid
Skydaekke
Uvejr
Kegle til Stormvarsel
Nedsynken
Sommer
(1) Solskive (2) Solur
Sol-S0jle
Solopgang, Solnedgang
Morgenraden Aftenreden
Sunshine
Sunspot number
Supersaturation
Surface of discontinuity
Solskin
Solpletantal
Overmaetning
Diskontinuitetsflade
DUTCH
Rug
Rijp
Westenwind
Rol-cumulus
Zoutgehalte
Zandpilaar
Sastrug
Verzadiging
(Bergnevel)
Stevenson's hut
Flarden, rafels
Zeewind
Zeegang
Zeeniveau
Jaargetijden
Seculaire gang
Seismen, aardbevingen
Helder
Temperatuur in de schaduw
Stortbui
Ruige vorst
Ligging
Natte sneeuw
Rekenschuif of rekenliniaal
(voor loods-ballon)
Slingerthermometer
Sneeuw
Sneeuwjacht
Sneeuwgrens
Sneeuwbedekking
(Sneeuwrollen)
Losse hagel
Solarisatie
Zonnestilstand
Registreerballon
Lente
Windvlaag
Buienlijn
Standaard, normaal
Middelbare fout
Standaard tijd
Bewolking, hemelbedekking
Storm
Stormkegel
Bezwijken, ineenzakken
Zomer
Zonnewijzer
Zuil boven de zon
Zonsopgang, ondergang
Morgenrood, avonrood
(schemeringskleuren)
Zonneschijn
Zonnevlekken getal
Overzadiging
Discontinuiteitsoppervlak
* American equivalent, frozen rain.
237
—continued
FRENCH
Crete de haute pression cm dorsale
Givre
Cumulus en rouleaux
Salinite
Trombe de sable
Sastrugi
Saturation
GERMAN
Riicken (beim Hoch)
Rauhreif
Brave Westwinde
Roll-Kumulus
Exposition
Neige et pluie melees (pas de terme
special en fran9ais)
Rdgle a calcul pour le d^pouillement des sondages aerologiques
Thermomdtre-fronde
Neige
Chasse-neige
Limite des neiges perpetuelles
Couverture de neige
Rouleaux de neige
GrSsil
Solarisation
Solstice
Ballon-sonde
Printemps
Grain
Ligne de grain
Standard, Etalon
Ecart moyen
Temps legal
Etat du ciel
Tempete
C6ne de Tempete
Subsidence
Ete
Cadran solaire
Colonne solaire
Lever, coucher du soleil
Couleurs du levant, du couchant
Salzgehalt
Staubsaule
Sastrugi
Sattigung
(Dicker Bergnebel mit Nieselregen)
Hiitte, Stevenson
Fractonimbus
Seebrise
Seegang
Meeresniveau
Jahreszeiten
Sakularschwankung
Seismische Bewegungen
Tau
Schattentemperatur
Schauer
Reif
Lage
Schnee und Regen gemischt
(Schlackenwetter)
Rechenschieber fiir Auswertung von
Pilotballonbeobachtungen
Schleuderthermometer
Schnee
Schneetreiben
Schneegrenze
Schneelage
Schneeroller
Graupel
Solarisation
Solstitium, Sonnenwende
Sondierballon
Friihling
Boe
Boenlinie
Standard, Normal
Mittlere Abweichung
Normalzeit
Himmelszustand (Menge derBewolkung)
Sturm
Sturmkegel
Absinken
Sommer
Sonnentagig
Sonnensaule
Sonnenaufgang, Sonnenuntergang
Dammerungsfarben
Insolation
Nombres des taches solaires
Sursaturation
Surface de discontinuity
Sonnenschein
Sonnenfleckenzahl
Ubersattigung
Diskontinuitatsflache
Abri
Diablotin, Nuage coureur, " Scud "
Brise de mer
Agitation de la mer
Niveau de la mer
Saisons
Variations seculaires
Seismes
Serein
Temperature sous abri
Averse
238
ENGLISH
Surface of subsidence
Surge
Swell
DANISH
Subsidensflade
D0nning
LIST OF EQUIVALENTS
DUTCH
Vlak van geringsten weerDrukgolf
[stand
Deining
Tension of vapour
Terrestrial magnetism
Thaw
Thunder
Thundercloud
Thunderstorm
Tide, Influence on winds
Time
Trace (of rainfall)
Trade winds
Trajectory
Transparency
Tremor
Tropic
Trough
Twilight
Twilight arch
Typhoon
Tid
Spor
Passat
Bane
Gennemsightighed
Skaelv
(1) Troperne (2) Vendekreds
Udk»ber af lavt Lufttryk
Tusmorke
Jordskygge
Tyfon
Units
Upper air
Enheder
H0jere Luftlag
Valley breeze
Vapour pressure
Veering
Dalvind
Damptryk
Vinden drejer til h0jre
Eeenheden
Bovenlucht, hoogere
luchtlagen
Dalwind
Dampdruk
Ruimend
Velocity
Vernier
Visibility
Water
Watershed
Waterspout
Water vapour
Wave motion
Waves
Weather
Weather maxim
Hastighed
Nonius
(1) Synsvidde (2) Sigtbarhed
Vand
Vandskel
Skypumpe
Vanddamp
B01gebevaegelse
(1) B01ger (2) S0
Vejr
Vejrregel
Snelheid
Nonius
Zicht
Water
Waterscheiding
Waterhoos
Waterdamp
Golfbeweging
Golven
Weer
Weerregel
Wedge
Weighting
Wet bulb
Wet day
Wet fog
Whirlwind
Wind
Wind vane
Winter
Wireless telegraphy
Kile
Tillaegge Vaegt
Vaadt Termometer
Wig
Afwegen
Natte bol
Natte dag, regendag
Natte mist
Dwarrelwind, wervelwind
Wind
Windvaan
Winter
Draadlooze Telegraphic
Year
Zero
Zodiac
Zone of silence
Zone time
Damptryk
Jordmagnetisme
T0
Torden
Tordensky
Tordenvejr, Tordenbyge
Spanning van den damp
Aardmagnetisme
Dauw
Donder
Donderwolk, onweerswolk
Onweer
Getij, invloed op wind
Tijd
Spoor (regen)
Passaatwinden
Baan
Doorzichtigheid
Tremor (siddering, trilling)
Keerkring
Dal, trog
Schemering
Schemering boog
Typhoon
vaad Taage
Hvirvelvind
Vind
Vindfl0j
Vinter
Radiotelegrafi, Traadl0s
Telegrafi
Aar
Jaar
(1) Nul (2) Nulpunkt
Nul
Dyrekredsen
Dierenriem
Tavshedszone
Stiltezone
Zonetid
Zone-tijd
239
—continued
FRENCH
Surface de subsidence
Variation generate
Houle
GERMAN
Abgleitflache
Allgemeines Ansteigen des Luftdrucks
Schwall, Diinung
Tension de Vapeur
Magnetisme terrestre
Degel
Tonnerre
Nuage d'orage
Orage avec tonnerre
Maree, Influence sur les vents
Temps (dur6e)
Traces de pluie
Vents alizes
Trajectoire
Transparence
Vibration
Tropique
Creux
Cr6puscule
Arc crepusculaire
Typhon
Dampfdruck
Erdmagnetismus
Tau
Donner
Gewitterwolke
Gewittersturm
Flut, Einfluss auf Winde
Zeit
Spur, Unmessbarer Niederschlag
Passatwinde
Trajektorie, Luftbahn
Transparenz, Durchlassigkeit
Beben, Zittern
Wendekreis
Troglinie (der Depression)
Dammerung (Zwielicht)
Dammerungsbogen
Taifun
Unit6s
Haute atmosphere
Brise de vallee
Pression de vapeur
Mouvement dextrogyre
Virement ou virage
Vitesse
Vernier
Visibilite
Eau
Versant
Trombe marine
Vapeur d'eau
Mouvement en vague
Ondes
Temps (m6t6orologique)
Proverbe meteorologique
Einheiten
Obere Luftschichten, Hohe (hohere)
Atmosphare
Talwind
Dampfdruck
Ausschiessen (des Windes)
Coin de haute pression
Pesant
Thermometre mouille
Jour humide
Brouillard humide
Tourbillon de vent
Vent
Girouette
Hiver
Telegraphic sans fil
Geschwindigkeit
Nonius
Sicht
Wasser
Wasserscheide
Wasserhose
Wasserdampf
Wellenbewegung
WeUen
Wetter
Wetterregel, Wetterspruch,
Volkswetterregel, Bauernregel
Keil (Hochdruckkeil)
Gewicht
Feuchtes Thermometer
Regentag
Nassender Nebel
Wirbelwind
Wind
Windfahne
Winter
Drahtlose Telegraphic
Annee
Zero
Zodiaque
Zone de silence
Temps du fuseau
Jahr
Null, Nullpunkt
Zodiakus, Tierkreis
Zone des Schweigens
Zonenzeit
240
LIST OF
The following pages give the equivalents in various languages of a numb
er of the terms
languages, and these have been omitted from the lists. The compl
ete list of translations
ENGLISH
ITALIAN
NORWEGIAN
Absorption (atmospheric) Assorbimento (atmosferico)
Absor
psjon
(Luftens)
Accumulated temperature Temperatura accumulata
Temperaturoverskudd
Accuracy
Accuratezza
N0iaktighet
Actinic rays
Raggi attinici
Aktiniske straler
Afterglow
Tramonto purpureo (del sole) Eftergl0d
Air
Aria
Luft
Air-meter
Misuratore di aria
Vindhastighetsmaler,
Anemometer
Air pockets
Tasche di aria
Lufthuller
Air trajectory
Traiettoria dell'aria
Lufttrajektor, Luftmasses
Altimeter
Altimetro
H0idemaler
[bane
Altitude
Altitudine
(1) H0ide (2) H0idevinkel
Anabatic
Anabatico
Anabatisk
Anemogram
Anemogramma
Anemogram
Anemometer
Anemometro
Vindhastighetsmaler,
Anemometer
Anemoscope
Anemoscopio
Anemoskop
Anthelion
Antelio
Motsol
Anticyclone
Anticiclone
Anticyklon
Antitrades
Contro alisei
Antipassat
Anvil cloud
Nube a incudine
Amboltsky
Aqueous vapour
Arid
Atmosphere
Atmospheric pollution
Atmospheric pressure
Atmospherics
Vapor acqueo
Arido
Atmosfera
Polluzzione atmosferica
Pressione atmosferica
Atmosferici
Attached thermometer
Audibility
Aurora
Autumn
Avalanche wind
Average
Termometro unito al
Barometertemometer
Barometro
Audibilita
H0revidde
Aurora
Polarlys, Nordlys, Sydlys
Autunno
H0st
Vento a valanga
Skred-vind
Media
Middel
Backing
Ball lightning
Balloon
Bearing
Beaufort notation
Beaufort scale of wind
force
Black-bulb thermometer
Salto di vento
Fulmine globulare
Pallone
Azimut
Indicazione di Beaufort
Scala di Beaufort della forza
del vento
Termometro a bulbo annerito
Blizzard
Blizzard (vento con
(1) Blizzard
nevischio) (2) Snekave, Snestorm
Pioggia di sangue
Blodregn
Bleu del cielo
Himlens bla farve
Brava vento di ovest
Nordlige vestenvindsbelte
Brezza
Bris*
Turbolenza dell'aria
Hiving
Leggerezza
Opdrift
Blood-rain
Blue of the sky
Brave West Winds
Breeze
Bumpiness
Buoyancy
Vanndamp
Arid, 0rkenAtmosfaere
Forurensning av luften
Lufttrykk
Luftelektriske forstyrrelser
Venstredreiende
Kulelyn
Ballong
Peiling
Beauforts vserkode
Beauforts vindskala
Sortkuletermometer
* Light breeze, svak vind; strong breeze, liten kuling.
241
EQUIVALENTS
defined in the Glossary. Many of the words were found to be practically identical in all
are available for reference in the Meteorological Office Library.
SWEDISH
PORTUGUESE
SPANISH
Absorption (Atmosfarens)
Absorci6n atmosfdrica
Abs6r5ao atmosferica
Varmesumma
Temperatura acumulada
Temperatura acumulada
Precisi6n
Nogrannhet
Exatidao
Fotografiskt verksamt ljus
Rayos actinicos
Raios actinicos
Efterglod
Resplandor montanero
Fen6menos 6pticos
(2a fase)
durante o crepusculo
Luft
Aire
Ar
Kanslig vindmatare
Anem6metro de mano
Aspirador graduado
aereos
Traject6ria do ar
Altimetro
Altitude
Anabatico
Anemodiagrama
Anemometro
Pozos de aire
Trayectoria del aire
Altimetro
Angulo de altura
Anabatico
Anemograma
Anem6metro
Anemoscopio
Anemoscopio
Antihelio
Antelio
Anticicl6n
Anticiclone
Contraalisio
Contra-moncao
Nube en yunque
Nuvem em forma de
bigorna
Vapor acuoso
Vapor aquoso
Arido
Arido
Atm6sfera
Atmosfera
Impureza atmosf6rica
PoluQao atmosferica
Presion atmosferica
Press ao atmosf erica
Atmosfericos
Atmosf6ricos
Luftgropar
Luftpartikelbana
Hojdmatare
Hojdvinkel
Anabatisk
Vindregistrerin g
Anemometer, vindmatare
Anemoskop, vindfana-flojel
Motsol. Antheli
Anticyklon. Hogtryck
Antipassad
Stad-moln
Term6metro adjunto
Term6metro unido
Vattenanga
Arid, okenLuftkrets. Atmosfar
Luftens fororening
Lufttryck
Atmosfariska storningar.
" Luftis "
Termometern pa barometern
Audibilidade
Aurora
Outono
Rajada de vento
Media
Audibilidad
Aurora
Otofio
Viento del alud
Promedio
Horbarhet
Norrsken, Sydsken, Polarsken
Host
Fallvind vid snoskred
Medeltal
De sentido retrograde
Relampago esferico
Balao
Rumo
Notacao Beaufort
Escala Beaufort, da for9a
do vento
Term6metro de
reservatbrio negro
" Blizzard "
Rolada a la izquierda
Rayo en bola
Globo
Rumbo nautico
Notaci6n de Beaufort
Escala de Beaufort
Motsolsvridning
Klotblixt
Ballong
Baring
Beauforts beteckningar
Beauforts vindskala
Term6metro de bola negra
Svartkuletermometer
Ventisca
Blizzard
Chuva de sangue
Azul do ceu
Ventos fortes de W
Brisa
Lluvia de sangre
Azul del cielo
Vientos generales del oeste
Brisa
Meneo
Boyanza
Blodregn
Himmelens blaa f arg
Carga de flutuacao
Bris
Gropighet, byighet
Fri lyftkraft
242
ENGLISH
LIST OF EQUIVALENTS
ITALIAN
Calibration
Calm
Cap
Catchment area
Clear sky, Day of
Climatic changes
Calibrazione
Calma
Cappa
Area di presa
Giprno di cielo chiaro (sereno)
Cambiamenti di clima
Cloud-burst
Cloudiness
Clouds
Col
Spiraglio fra le Nubi
Annuvolamento
Nube
Col (regione di media
pressione)
Fronte fredda
Onda fredda
Bussola
Conduzione
Corona
Raggi crepuscolari
Curva idonea
Cold front
Cold wave
Compass
Conduction
Corona
Crepuscular rays
Curve fitting
Damp air
Dawn
NORWEGIAN
Kalibrering
Vindstille
Hette, Skykappe
Bekken
Klar dag
KUmaendringer
Skybrudd
Skydekke
Skyer
Sadel
Kaldfront
Kuldeb01ge
Kompass
Ledning
Korona, Krans
Tusm0rkestraler
Utjevning ved kurve
Aria umida
Alba
Giorno
Disgelo
Densita
Depressione
Deserto
Desiccazione
Deviazione
Rugiada
Diatermano
Klam luft
Daggry
(1) Dag (2) D0gn
Isgang
Tetthet
Lavtrykk, Lavtrykksomrade
0rken
Utt0rring
Deviasjon
Dynamic cooling
Diatermansi, Gjennemstralelighet for varne
Diffrazzione
Diffraksjon, b0ining
Diurno
Daglig
Zona di calma equatoriale
Doldrum, Ekvatoriale
stillebelter
Spruzzatore
Yr, Duskregn
Goccia
Draper
Siccita
T0rke
Periodo di siccita
T0rkeperiode
Durata dello splendore del sole Varighet av solskinn
Crepuscolo
Tusm0rke, Skumring
Pulviscolo
St0v
Vento vorticoso (in regione
St0vhvirvel
sabbiosa e secca)
Raffreddamento dinamico
Dynamisk avkj01ing
Earth currents
Earthquake
Eddy
Equation of time
Equatorial air
Equinox
Error
Evaporation
Expansion
Corrente terrestre
Terremoto
Piccolo vortice
Equazione del tempo
Aria equatoriale
Equinozio
Errore
Evaporazione
Espansione
Jordstrammer
Jordskjelv
Bakevje turbulent
Tidsjevning
Tropeluft
Jevnd0gn
Feil
Fordunstning
Utvidelse
Day
Debacle
Density
Depression
Desert
Desiccation
Deviation
Dew
Diathermancy
Diffraction
Diurnal
Doldrums
Drizzle
Drops
Drought
Dry spell
Duration of sunshine
Dusk
Dust
Dust-devil
243
—continued
PORTUGUESE
SPANISH
Calibrate
Calma
Lenticular, barrete
Area de captacao
Dia de ceu limpo
Varia9oes climateiicas ou
climaticas
Batega de agua
Nebulosidade
Nuvens
Sela, garganta
Calibraci6n
Calma
Toca
Cuenca
Dia despejado
Cambios climaticos
SWEDISH
Kalibrering, profning
Lugnvader. Blekstillt
Molnhatta
Flodomrade
Dag med klar himmel
Klimatandringar
Turbi6n
Nubosidad
Nubes
Collado
Skyfall
Molnighet
Moln
Sadel
Frente fria
Vaga de Mo
Bussola
Conducao
Coroa
Raios crepusculares
Curva ou linha
representativa
AT humido
Aurora
Dia
Desgelo
Densidade
Depressao
Deserto
Secura
Desvio
Orvalho
Diatermicidade
Frente frio
Ola de frio
Compas de bitacora
Conducci6n
Corona
Rayos crepusculares
Adaptaci6n grafica
Kallfront
Koldvig
Kompass
Ledning, varmeledning
Krans
Skymningsstralar
Anpassning af en kurva
Aire hfimedo
Aurora
Dia
Gran desbielo
Densidad
Depresi6n
Desierto
Desecaci6n
Desviaci6n
Rocio
Diatermancia
Fuktig luft
Gryning
Dygn
Islossning
Tathet
Minimum, lagtryck
Oken
Uttorkning
Afvikelse
Dagg
Diatermansi
Difragao
Diurno
Regioes equatoriais de
calmas
Chuvisco
Gotas
S6ca
Periodo sSco
Duracao do brilho do sol
Crepusculo
Poeira
Turbilhao de poeira
Difracci6n
Diurno
Zona de calmas ecuatoriales
DifEraktion, bojning
Daglig
Ekvatoriala kalmerna
Llovizna
Gotas
Sequia
Periodo seco
Horas de insolaci6n
Anochecer
Polvo
Tolvanera
Dugg. Duggregn
Droppar
Torka
Torr period
Antal solskenstimmar
Skymning (Astronomisk)
Stoft
Damm-hvirfvel
ResMamento dinamico
Enfriamiento dinamico
Dynamisk afkylning
Correntes teluricas
Tremor de terra
Remoinho
Equa9ao do tempo
Ar equatorial
Equinoxio
Erro
Evapora9ao
Expansao
Corrientes teluricas
Temblor de tierra
Remolino
Ecuaci6n de tiempo
Aire ecuatorial
Equinoccio
Error
, Evaporaci6n
Dilataci6n
Jordstrommar
Jordskalf. Jordbafning
Hvirfvel
Tidsekvation
Ekvatorial-luft
Dagjamning
Fel
Avdunstning
Utvidgning
244
ENGLISH
ITALIAN
Esposizione
Essicazione
Estremi
Occhio della tempesta
Occhio del vento
Exposure
Exsiccation
Extremes
Eye of storm
Eye of wind
LIST OF EQUIVALENTS
NORWEGIAN
Eksponering, opstilling
Uttorring
Ekstremer
Stonruaie
Vindoie
Barometrets falletid
Freeze, Freezing
Frequency
Friction
Front
Frost
Funnel cloud
Further outlook
Tempo di discesa del
barometro
Temperatura per la riduzione a
O° delle letture barometriche
Mare ghiacciato
Deflusso
Nebbia
Arcobaleno bianco
Previsione
Ghiaccio in aghetti o
piastrine
Gelare, congelamento
Frequenza
Frizione
Fronte
Brina
Nube a imbuto
Previsione a lunga scadenza
Frost
Frekvens, Hyppighet
Friksjon
Front
Frost
Skypumpe
Langtidsvarsel
Gale
Gale warning
General inference
Vento fresco
Avviso di vento fresco
Inferenza generale
Sterk kuling*
Kulingvarsel, Stormvarsel
Vaeroversikt
Glacier breeze
Glazed frost
Brezza glaciale
Gelicidio
Brevind
Isslag
Grass temperature
Gravity
Great circle
Green flash
Ground frost
Ground, State of
Temperatur i gresset
Tyngdekraft
Storcirkel
Den gr0nne strale
Gust
Temperatura del suolo erboso
Gravita
Circolo massimo
Raggio verde
Brina sul terreno (suolo gelato)
Terreno (superncie della
terra), stato del
Colpo di vento
Hail
Halo
Grandine
Alone
Haze
Heat
High
Hoar-frost
Horse latitudes
Humidity
Hurricane
Hydrometer
Nebbia (secca), caligine
Galore
Alta (pressione)
Brina
Latitudine delle calme
Umidita
Uragano
Idrometro
Falling time (of
barometer)
Fiducial temperature
Floe
Flow-off
Fog
Fog bow
Forecast
Frazil ice
Barometrets referenstemperatur
Isflak
Avl0p
Take
Takebue, Hvit regnbue
Vaervarsel
Markens tilstand
Rosse, Vindkast, Vindst0t
Hagl
Halo, Ring, Vaergard:—
(1) Solgard
(2) Manegard
01r0k, Moe
Varme
H0itrykk, H0itrykksomrade
(1) Rim (2) Takerim
Hestebredder
Fuktighet
Orkan
Areometer
* Beaufort force 8; force 7, stiv kuling; force 9, liten storm
245
—continued
SWEDISH
PORTUGUESE
Exposigao
Dissecagao, Exsicagao
Extremas
Olho da tempestade
Ponto irradiante
SPANISH
Instalaci6n
Exsecaci6n
Extremas
Ojo de la tempestad
Filo del viento
Utstallning
Uttorkning
Extremer
Stormens oga
Mot vinden
Morosidade do bar6metro
Inercia del bar6metro
Installningstid
Temperatura fiducial
Temperatura compensadora
Blocos de gelo flutuantes
Area de inundacao
Nevoeiro
Arco de nevoeiro
Previsao
Gelo frazil
Tempano
Caudal
Niebla
Arco de la niebla
Predicci6n
Chispas de hielo
Temperatur, da barometern
visar ratt
Isflak
AMnning
Dimma
Dimregnbage
Vaderleksforutsagelse
Tallriksis
Geada, glacial
Frequencia
Fricgao
Frente
Geada
Nuvem em funil
Previsao prolongada
Helar, Helada
Frecuencia
Rozamiento
Frente
Escarcha
Nube en embudo
Indicaci6n ulterior
Frysa. Fryspunkt
Frekvens
Friktion
Front
Frost
Skorstensmoln
Utsikter for i ofvermorgon
Vento forte
Aviso de vento forte
Inferencia ou conclusao
geral
Brisa glaciarica
Geada vidrada ou
prateada
Temperatura da relva
Gravidade
Circulo maximo
Raio verde
Geada no terreno
Estado do terreno
Temporal
Aviso de temporal
Inferencia general
Hard vind eller storm
Stormvarning
Sammanfattning
Brisa de glaciar
Lluvia helada
Jokelvind
Isbark
Temperatura junto al suelo
Gravedad
Circulo maximo
Rayo verde
Helada
Estado del terreno
Grastemperatur
Tyngdkraft
Storcirkel
Grona stralen
Frost med skador
Markens tillstand
Rajada
Rafaga, Racha
By
Saraiva
Halo
Granizo
Halo
Hagel
Halo
Cerracao
Calor
Altura
GeMa branca
Regioes de calma
Humidade
Furacao
Hidr6metro
Calina, Calima
Calor
Alta
Escarcha
Zona de calmas tropicales
Humedad
Huracan
Hidrodensfmetro
Disa
Varme
Hogtryck
Rimfrost
Rossbredderna
Fuktighet
Orkan
Areometer
246
ENGLISH
ITALIAN
LIST OF EQUIVALENTS
NORWEGIAN
Ice sheet
Increment
Index correction
Indian summer
Ghiaccio
Is
Iceberg (montagna di
Isfjell, Isberg
ghiaccio)
Macchia luminosa nel cielo
Isblink
sopra il ghiaccio
Coperta di ghiaccio (lenzuolo) Innlandsis
Tilvekst
Incremento
Nullpunktskorreksj on
Indice delle correzzioni
Indian summer
Estate indiana
Isopycnic
Superfici di uguale densita
Isopyknisk
Katabatic
Kite
Katabatico
Cervo-volante
Fall (vind)
Drage
Labile
Lag
Land and sea breezes
In equilibrio instabile
Pigro
Brezze di terra e di mare
Ice
Iceberg
Iceblink
Latitude
Level
Lightning
Line-squall
Labil
Innstillingstid
Solgangsvind :—
(1) Landbris,
(2) Sj0bris
Decremento della temperatura Vertikal temperaturgradlent
con 1'altezza
Bredde
Latitudine
Niva
Livello
Lyn, Kornmo*
Lampo
Linjebyge
Linea della tempesta
Liquid
Local time
Longitude
Looming
Liquido
Tempo locale
Longitudine
Ingrandimento apparente
Vaeske
Lokaltid
Lengde
Forst0rrelse ved take
Low
Lunar
Profondo
Lunare
Lavtrykk, Lavtrykksomrade
Mane-
Mackerel sky
Mares' tails
Mercury
Mirage
Mist
Monsoon
Month
Moon
Mountain breeze
Cielo con nubi ondulate
Code di cavallo (nube)
Mercurio
Miraggio
Nebbia
Monsone
Mese
Luna
Brezza di montagna
Makrellskyhimmel
Meiskyer, Revehaler
Kvikksolv
Luftspeiling, Hildring
Takedis
Monsun
Maned
Mane
Fj ellvind
Night sky, Light of
Normal law of errors
Luminosita del cielo di notte Nattehimlens lys
Legge normale degli errori
Feilloven
Observer
Oscillation
Overcast day
Osservatore
Oscillazzione
Giorno caliginoso
Lapse
* Distant lightning.
Observat0r, iakttager
Oscillasjon, Svingning
Overskyet dag
247
—continued
PORTUGUESE
SWEDISH
SPANISH
Hielo
" Iceberg." Montafia flotante
de hielo
Resplandor del hielo
Is
Isberg
Ismassa
Inkrement
Indexkorrektion
Indiansommar
De igual densidade
Sabana de hielo
Incremento
Correci6n de indice
Veranillo de San Martin,
Veranillo del membrillo
Isoplcnica
Catabatico
Papagaio
Catabatico
Cometa
Katabatisk
Drake
Labil (instavel)
Atraso ou demora
Brisas da terra e do mar
Labil
Labil
Efterslapning
Retardo
Land-och sjobris
Brisas de tierra y de mar.
(Terra! y viraz6n)
Gelo
" Iceberg "
" Iceberg "
Len9ol de gelo
Acrescimo
Correccao do index
Verao de S. Martinho
Deminuicao da tempera- Degradaci6n de temperatura
tura com a altura
Latitud
Latitude
Nivel
Nivel
go
Relampa
Relampago, raio
turbonada
de
Linea
ou
,
Linha de borrasca
duma
Ma
frente
depressao
Liquido
Liquido
Hora local
Hora local
Longitud
Longitude
s de calina, Espejismo
Espectro
Amplificacao aparente
de calina
Baja
Baixo
Lunar
Lunar
Isblink
Isodens
Vertikalgradient
Latitud. Bredd
Vagrat. Niva
Blixt
Linjeby
Vatska
Ortstid
Longitud. Langd
Hagring
Lagtryck
Man-
Ceu cinzento
Caudas de cavalo
Mercurio
Miragem
Nevoa, nebrina
Mongao
Mes
Lua
Brisa da montanha
Cielo aborregado
Rabos de gallo, colas de gato
Mercurio
Espejismo
Neblina
Monz6n
Mes
Luna
Brisa de montana
Makrihnohi
Hastsvansar
Kvicksilfver
Hagring
Disa
Monsun
Manad
Mane
Berg-och dalvind
Luz do c6u A, noite
Lei normal dos erros
Claridad del cielo nocturno
Ley normal de los errores
Natthimlens ljus
Gauss' fellag
Observador
Oscila9§,o
Dia enevoado
Observador
Oscilaci6n
Dla cubierto
Observator
Svangning.
Mulen dag
Variation
248
ENGLISH
LIST OF EQUIVALENTS
NORWEGIAN
Pakkis
Pack ice
Pallium
Paranthelia
Paraselanae
Parhelion
Passing shower
ITALIAN
Ghiaccio ammonticchiato
Pallium (nube)
Parantelio
Paraselenio
Parelio
Pioggia di breve durata
Percolation
Persistence
Percolazione
Persistenza
Jevnt skylag
Motsol
Bimaner
Bisol
Vandrende byger,
Vandrende skurer
Nedsiving
Vedvaren
Personal equation
Phenomenon
Pilot balloon
Pitot tube
Pluviometer
Pocky cloud
Polar air
Precipitation
Pressure
Probability
Probable error
Prognostics
Pumping
Purple light
Radiation
Rain
Rain, Artificial
Rain-band
Equazione personale
Fenomeno
Pallone pilota
Tubo di Pitot
Pluviometro
Nube a tasca
Aria polare
Precipitazione
Pressione
Probabilita
Errore probabile
Prognostic!
Instabilita (barometro)
Luce purpurea
Radiazione
Pioggia
Pioggia artificiale
Banda della pioggia
Personlig feil
Fenomen, foreteelse
Pilotballong
Pitotr0r
Pluviometer, Regnmaler
MammatoPolarluft
Nedb0r
Trykk, Lufttrykk
Sannsynlighet
Sannsynlig feil
Vaermerker, Vaerregler
Pumpning
Purpurlys
Straling
Regn
Kunstig regn
Regnband
Rainbow
Rain day
Raindrops
Rainfall
Rain-gauge
Rain-making
Rain shadow
Rain-spell
Recurvature of storm
Arcobaleno
Giorno di pioggia
Gocce di pioggia
Caduta di pioggia
Pluviometro
Pioggia artificiale
Area di poca pioggia
Lungo periodo di pioggia
Deviazione della tempesta
Reduction
Reflection
Refraction
Residual
Reversal
Revolving storm
Ridge
Rime
Roaring Forties
Riduzione
Riflessione
Rifrazione
Residue
Rovesciamento (del vento)
Ciclone tropicale
Sommita
Nebbia gelata
Roll cumulus
Roll-cumulus
Regnbue
Dag med nedbor> 0.2 mm.
Regndraper
Nedb0r
Regnmaler
Regnmakeri
Le for,nedb0r
Regnvaersperiode
Tilbakeb0ining av tropisk
stormbane
Reduksjon
Refleksjon, Speiling
Refraksjon, Brytning
Awikelse fra utjevnet verdi
Vindomslag med h0iden
Hvirvelstorm
Kile, Rygg
(1) Takerim (2) Run
Sydlige vestenvindsbelte,
De brave vestenvinde
Skyruller
Salinity
Sand pillar
Sastrugi
Saturation
Scotch mist
Salinita
Ondulazioni sulla neve
Saturazione
Precipitazione da nebbia fine
Saltgehalt, Saltholdighet
Sandhvirvel
Skavler
Metning
Takeyr
249
—continued
PORTUGUESE
Pacote de gelo
" Pallium "
Parantelio
Paraselena
Parh61io
Aguaceiro passageiro
SPANISH
Banco de hielos a la deriva
Palio. (Pallium)
Paranthelios
Paraselenes
Parhelio
Chubascos pasajeros
Filtracao
Persistencia
Filtraci6n
Persistencia
Ecuaci6n personal
Fen6meno
Globo piloto
Tubo de Pitot
Pluvi6metro
Nubes abolsadas
Aire polar
Precipitaci6n
Presi6n
Probabilidad
Error probable
Pron6sticos
Tembleteo
Luz purpurea
Radiaci6n
Lluvia
Lluvia artificial
Banda de absorci6n de la
lluvia
Arco iris
Arco-lris
Dla de lluvia
Dia de chuva
Gotas de lluvia
Pingos de chuva
Precipitaci6n
Chuva calda
Pluvi6metro
Pluvidmetro
Pluvificaci6n
Chuva artificial
Area protegida das chuvas Sequia orografica
Periodo lluvioso
Periodo chuvoso
Incurvaci6n de la tempestad
Encurvamento da
tempestade
Reducci6n
Reduccao
Reflexi6n
Reflexao
Refraccidn
Refraccao
Desviaci6n
Residual, restante
Reversi6n
Reversao
Cicl6n, tif6n, huracan, baguio
Ciclone, furacao
Loma
Crista
Cencenada (Niebla helada)
Nevada
Ponientes duros australes
Equacao pessoal
Fen6meno
Balao piloto
Tubo de Pitot
Pluvi6metro
Nuvem bexigosa
AT polar
Precipitacao
Pressao
Probabilidade
Error provavel
Prognosticos
Oscilacao
Luz purpurea
Radiagao
Chuva
Chuva artificial
Risca de chuva
Cumulus em forma de r61o
Salinidade
Coluna de areia
" Sastrugi"
Saturacao
Nebrina da Escocia
Rollo-cumulos, 6 cumulos en
rollo
Salinidad
Tolvanera
" Sastrugi "
Saturaci6n
Neblina escocesa
SWEDISH
Packis
Regnmolnstacke
Motbisol. Parantheli
Bimane
Bisol. Parheli
Overgaende skur
Perkolation
Persistens.
" Erhaltungstendenz '*
Personlig ekvation
Fenomen
Pilotballong
Pitotror
Regnmatare
MammatoPolarluft
Nederbord
Tryck. Lufttryck
Sannolikhet
Sannolikt fel
Vaderlekstecken
Pumpning
Purpursken
Straining
Regn
Konstgjordt regn
Regnband
Regnbage
Nederbordsdag
Regndroppar
Nederbord
Nederbordsmatare
Regnmakeri
Regnskugga
Regnperiod
Stormens riktningsandring
Reduktion
Reflektion. Aterkastning
Refraktion. Brytning
Residu
Vindomkastning med hoj den
Hvirfvelstorm
Rygg af hogt lufttryck
Dimfrost
Vals-Cumulus
Salthalt
Sandstod
Skaflor
Mattning
Regndimma
250
LIST OF EQUIVALENTS
ENGLISH
Screen, Stevenson
Snow line
Snow lying
Snow rollers
Soft hail
Solarisation
Solstice
Sounding balloon
Spring
Squall
Squall line
NORWEGIAN
ITALIAN
Capanna (meteorologica di
Stevensons bur
Stevenson)
Fracto-nimbo
Brezza di mare
Sjobris
Burrasca marina
Sjogang
Livello del mare
Havsniva
Arstider
Stagione
Ciclo secolare
Sekulaer endring
Sismo
Jordrystelser
Sereno (pioggia con cielo)
Regn fra klar himmel
Temperatura all'ombra
Temperatur i skyggen
Skur, byge
Rovescio
Galaverna
Rimslag
Beliggenhet
Luogo
Pioggia gelata
Sludd
Regolo calcolatore per palloni Regnestav for pilotballonger
piloti
Termometro a fionda
Slyngetennometer
Sne
Neve
Neve gelata ammonticchiata
(1) Snefokk, Snedrev
(2) Snedrive
Limite delle nevi (perpetue)
Snegrense
Snedekke
Cih'ndri di neve
Grandine molle
Hagl*
Insolazione
Eksponering for sollys
Solstizzio
Solhverv
Pallone sonda
Registrerballong
Primavera
V&r
Tempesta
Vindbyge, Byga
Fronte della tempesta
Bygelinje
Standard
Standard deviation
Standard time
State of the sky
Storm
Di ugual misura, campione
Deviazione tipo
Tempo medio locale
State del cielo
Burrasca, temporale
Scud
Sea breeze
Sea disturbance
Sea level
Seasons
Secular trend
Seisms
Serein
Shade temperature
Shower
Silver thaw
Site
Sleet
Slide rule, Pilot balloon
Sling thermometer
Snow
Snow drift
Storm cone
Subsidence
Summer
Sun-dial
Sun pillar
Sunrise, sunset
Standard
Middelfeil
Normaltid
Skydekke
(1) Uvaer, (2) Storm
Segnale di tempesta
Subsidenza
Estate
Orologio solare
Colonna solare
Sorgere del sole, tramonto
del sole
Sunrise and sunset colours Colori del sorgere e del
tramonto del sole
Sunshine
Splendore del sole
Sunspot number
Numero delle macchie solari
Stormvarselskj egle
Subsidens, Nedsynkning
Sommer
Solur
Sols0ile
Solopgang, Solnedgang
Supersaturation
Surface of discontinuity
Soprasaturazione
Superficie di discontinuity
Overmetning
Diskontinuitetsflate
Surface of subsidence
Superficie di subsidenza
Subsidensflate
Morgenr0den, Aftenr0den
Solskinn
Solflekktall
* Komsne is used for granular snow.
—continued
251
PORTUGUESE
Abrigo Stevenson
SPANISH
Garita de Stevenson
SWEDISH
Stevenson's bur
Fracto-nimbus
Brisa do mar
Perturbacao do mar
Nivel do mar
Estates
Variacao secular
Sismo
Sereno
Temperatura a sombra
Aguaceiro
Degelo prateado
Situacao
Geada miuda
Regua de calculo para
baloes pilotos
Term6metro funda
Neve
Neve amontoada
Correos
Viraz6n
Estado del mar
Nivel del mar
Estaciones del ano
Tendencia secular
Sismos
Sereno, relente
Temperatura a la sombra
Chubasco
Escarcha plateada
Situaci6n
Aguanieve
Regla de calculo para globos
pilotos
Term6metro honda
Nieve
Ventisquero y ventisca
Molnflagor
Sjobris
Sjogang
Havsytan
Arstider
Sekularvariation
Seismer
Regn fran klar himmel
Temperatur i skuggan
Skur
Glattis. Silfverdagg
Forlaggning
Snoblandadt regn
Raknesticka for ballongviseringar
Slungtermometer
Sno
Drifsno
Linha de neve
Limite de las nieves perpetuas
Neve jacente
La nieve cubre el suelo
Rolos de neve
Rollos de nieve
Graniso
Nieve granulada
Exposicao ao sol
Insolaci6n
Solsticio
Solsticio
Balao sonda
Globo sonda
Primavera
Primavera
Borrasca
Turbonada
Linha de borrasca ou Linea de turbonada
frente fria duma
depressao
Padrao
Patr6n, tipo, norma, modelo
Desvio da normal
Desviaci6n tipo
Hora normal
Hora civil
Estado do ceu
Nubosidad
Tempestade
Tempestad, temporal,
borrasca
Cone, aviso de tempestade Senales de tempestad
Abatimento
Subsidencia
Verao
Verano
Relojio de sol
Cuadrante solar
Coluna solar
Columna solar*
Nascer do sol e por do sol Orto, ocaso
Coloraciones del orto y ocaso
Colora9oes ao nascer e
p6r do sol
Horas de sol
Brilho do sol
Numeros de Wolf
Numero de manchas
solares
Sobresaturaci6n
Sobressaturacao
Superficie de discontinuidad
Superficie de
descontinuidade
Superficie de abatimento Superficie de subsidencia
* Also obelisco luminoso
Snograns
Snotacke
Snobultar
Trindsno
Utsattning for solljuset
Solstand
Ballon-sonde
Var
Stonnby. Stormil
Bylinje
Etalon
Medelafvikelse
Normaltid. Gemensam tid
Himmelens utseende
Storm
Stormvarningskon
Efterstromning
Sommar
Solvisare. Solur
Ljuspelare
Soluppgang. Solnedgang
Skymn i ngsfarger
Solsken
Solflackstal
Ofvermattning
Diskontinuitetsyta
Subsidensyta
252
ENGLISH
ITALIAN
LIST OF EQUIVALENTS
NORWEGIAN
Surge
Swell
Cambiamento di pressione
Onde (di mare) persistenti
Utstrakt trykkendring
D0nning
Tension of vapour
Terrestrial magnetism
Thaw
Thunder
Thundercloud
Thunderstorm
Tide, Influence on winds
Tensione del vapore
Magnetismo terrestre
Liquefazione
Tuono
Nube da tuoni
Temporale
Marea, influenza dei venti
Time
Trace (of rainfall)
Tempo
Traccia (della pioggia)
(piccola quantita)
Venti alisei
Traiettoria
Trasparenza
Tremito
Tropico
Vanndampens trykk
Jordmagnetisme
T0
Torden
Tordensky
(1) Tordenvaer (2) Tordenbyge
Tidevannets innfiydelse pa
vinden
Tid
Spor (av nedb0r)
Trade winds
Trajectory
Transparency
Tremor
Tropic
Trough
Twilight
Twilight arch
Typhoon
Crepuscolo
Arco crepuscolare
Tifone
Units
Upper air
Valley breeze
Vapour pressure
Veering
Velocity
Vernier
Visibility
Unita
Aria superiore
Brezza di valle
Tensione del vapore
Rotazione
Velocita
Verniero
Visibilita
Water
Watershed
Waterspout
Water Vapour
Wave-motion
Waves
Weather
Weather Maxim
Wedge
Weighting
Wet Bulb
Wet Day
Wet Fog
Whirlwind
Wind
Wind Vane
Winter
Wireless Telegraphy
Year
Zero
Zodiac
Zone of Silence
Zone Time
Acqua
Linea di displuvio
Tromba marina
Vapore di acqua
Moto ondoso
Onde
Tempo (meteorologico)
Aforismi sul tempo
Cuneo
Pesata
Bulbo bagnato
Giorno umido
Nebbia umida
Turbine
Vento
Anemoscopio
Inverno
Telegrafia senza fili
Anno
Zero
Zodiaco
Zona del silenzio
Fuso orario
Passat
Trajektor, Bane
Gj ennemsiktighet
Jordskjelv
(1) Vendekrets (2) Troperne
Lavtrykksrenne
Tusm0rke, Skumring
Jordskygge
Tyfon
Enheter
H0iere luftlag
Dalvind
Vanndampens trykk
H0iredreiende
Hastighet
Nonius
(1) Synsvidde (2) Siktbarhet
Vann
Vannskille
Skypumpe
Vanndamp
B01gebevegelse
B01ger
Veer
Vaermerke, Vaerregel
Kile, Rygg
Tildele vekt
Vatt termometer
Dag med nedb0r > 1 • 0 mm.
Vat take
Hvirvelvind
Vind
Vindfl0i
Vinter
Radiotelegrafi
Ar
(1) Null (2) Nullpunkt
Dyrekretsen
Taushetssone
Sonetid
253
—continited
PORTUGUESE
SPANISH
Variaci6n de conjunto
Mar de leva, Mar sordo,
Mar tendida
Tensao do vapor
Tensi6n del vapor
Magnetismo terrestre
Magnetismo terrestre
Degelo
Deshielo, fusi6n
Trueno
Trovao
Nuvem de trovoada
Nube tormentosa, nubarr6n
Tormenta
Trovoada
Marea, influencia sobre los
Mare, influencia sobre os
vientos
ventos
Hora (Tiempo)
Hora
Lluvia inapreciable
Vestigios de chuva
SWEDISH
Allman tryckstegring
Dyning
Angtryck
Jordmagnetiam
Aska
Askmoln
Askvader
Tidvattnets inflytande pa
vinden
Tid
Ngt (pronounced nagot)
Ventos regulares
Tragect6ria
Transparencia
Tremor
Tr6pico
Linha de baixa pressao
Crepiisculo
Arco crepuscular
Tufao
Unidades
Camada superior do ar
Briza do vale
Tensao de vapor
Sentido directo
Velocidade
N6nio
Visibilidade
Vientos alisios
Trayectoria
Transparencia
Temblor de tierra
Tr6pico
Vaguada
Crepusculo
Arco crepuscular
Tif6n o bagulo
Unidades
Atm6sfera superior
Brisa de valle
Tensi6n del vapor
Rolada a la derecha
Velocidad
Nonio
Visibilidad
Passader
Trajectorie
Genomskinlighet
Jordstotar
Vandkrets. Tropik
Trag
Skymning
Skymningsbage
Tyfon
Enheter
Hogre atmosfaren
Dalvind
Angtryck
Medsolsvridning
Hastighet
Nonie
Siktbarhet, synvidd
Agua
Aguas vertentes
Tromba de agua
Vapor de agua
Movimento ondulatorio
Ondas
Tempo
Maximas ou aforismos
Cunha
Peso
Reservat6rio molhado
Dia humido
Nevoeiro humido
Remofnho
Vento
Catavento
Inverno
Telegrafia sem fios
Agua
Divisoria
Tromba marina, manga
Vapor acuoso
Movimiento ondulatorio
Ondas, olas
Tiempo
Refran meteoro!6gico
Cufia
Ponderaci6n
Term6metro humedo
Dia de lluvia
Niebla humeda
Torbellino
Viento
Veleta
Invierno
Radiotelegrafla
Ano
Ano
Cero
Zodiaco
Zona de silencio
Hora del huso
Vatten
Vattendelare
Skydrag
Vattenanga
Vagrorelse
Vagor
Vader. Vaderlek
Vaderleksregel
As
Forse med vikter
Vata kulan
Regndag
Vatande dimma
Hvirfvelvind
Vind
Vindfana
Vinter
Tradlos telegrafi. Radio
Ar
Z6ro
Zodlaco
Zona de silencio
Fusos horarios
(90565) Wt. 3034/M.1315
K.15 8/50 Hw.
(T.S. 1610)
NoU
Djurkrets
Tyst zon
Zontid